cobalt: Tune CC GPU memory settings to reduce footprint (#7527)

This change tunes two GPU memory settings in the compositor to reduce
its overall memory footprint, particularly on Android devices.

The primary change reduces the amount of memory allocated for offscreen
content prepainting. On most Android devices, this was previously set to
use up to 67% of the compositor's memory budget for prepainting textures
outside the visible viewport. This is lowered to 25%.

Experimental results on an android-qa build on Sabrina show this change
significantly reduces the memory usage of the compositor (the 'cc'
component). When navigating quickly through video content, memory usage
dropped from ~70-80 MB to a more stable ~40-50 MB. Further local testing
showed that reducing this to 0% (disabling prepainting) could lower
usage to 30-40 MB with a minimal impact on user experience. We are
proceeding with 25% for now, but could consider further reductions in
the future.

Additionally, the GPU memory allocation priority was changed from
CUTOFF_ALLOW_NICE_TO_HAVE to CUTOFF_ALLOW_REQUIRED_ONLY. While this did
not show a significant memory reduction in testing, it is a logical
change for our use case, as scrolling is not a primary interaction on
most target devices, making "nice-to-have" allocations for scrolling
less critical.

It's worth noting that while the compositor's component memory
decreased, the peak RSS numbers reported by memory-infra did not show a
clear reduction. However, this metric is known to fluctuate. The impact
of these changes on overall system memory should be verified in
production.

Bug: 424591313

Author: sherryzy

third_party/blink/renderer/platform/widget/compositing/layer_tree_settings.cc

@@ -91,7 +91,11 @@ cc::ManagedMemoryPolicy GetGpuMemoryPolicy(
   cc::ManagedMemoryPolicy actual = default_policy;
   actual.bytes_limit_when_visible = 0;
   actual.priority_cutoff_when_visible =
+#if BUILDFLAG(IS_COBALT)
+      gpu::MemoryAllocation::CUTOFF_ALLOW_REQUIRED_ONLY;
+#else
       gpu::MemoryAllocation::CUTOFF_ALLOW_NICE_TO_HAVE;
+#endif
 
   // If the value was overridden on the command line, use the specified value.
   static bool client_hard_limit_bytes_overridden =
@@ -407,7 +411,11 @@ cc::LayerTreeSettings GenerateLayerTreeSettings(
     // On low-end we want to be very careful about killing other
     // apps. So initially we use 50% more memory to avoid flickering
     // or raster-on-demand.
+  #if BUILDFLAG(IS_COBALT)
+    settings.max_memory_for_prepaint_percentage = 0;
+  #else
     settings.max_memory_for_prepaint_percentage = 67;
+  #endif
   } else {
     // On other devices we have increased memory excessively to avoid
     // raster-on-demand already, so now we reserve 50% _only_ to avoid

third_party/blink/renderer/platform/widget/compositing/layer_tree_settings_unittest.cc

@@ -15,21 +15,33 @@ TEST(LayerTreeSettings, IgnoreGivenMemoryPolicy) {
   auto policy =
       GetGpuMemoryPolicy(cc::ManagedMemoryPolicy(256), gfx::Size(), 1.f);
   EXPECT_EQ(512u * 1024u * 1024u, policy.bytes_limit_when_visible);
+#if BUILDFLAG(IS_COBALT)
+  EXPECT_EQ(gpu::MemoryAllocation::CUTOFF_ALLOW_REQUIRED_ONLY,
+#else
   EXPECT_EQ(gpu::MemoryAllocation::CUTOFF_ALLOW_NICE_TO_HAVE,
+#endif
             policy.priority_cutoff_when_visible);
 }
 
 TEST(LayerTreeSettings, LargeScreensUseMoreMemory) {
   auto policy = GetGpuMemoryPolicy(cc::ManagedMemoryPolicy(256),
                                    gfx::Size(4096, 2160), 1.f);
   EXPECT_EQ(2u * 512u * 1024u * 1024u, policy.bytes_limit_when_visible);
+#if BUILDFLAG(IS_COBALT)
+  EXPECT_EQ(gpu::MemoryAllocation::CUTOFF_ALLOW_REQUIRED_ONLY,
+#else
   EXPECT_EQ(gpu::MemoryAllocation::CUTOFF_ALLOW_NICE_TO_HAVE,
+#endif
             policy.priority_cutoff_when_visible);
 
   policy = GetGpuMemoryPolicy(cc::ManagedMemoryPolicy(256),
                               gfx::Size(2048, 1080), 2.f);
   EXPECT_EQ(2u * 512u * 1024u * 1024u, policy.bytes_limit_when_visible);
+#if BUILDFLAG(IS_COBALT)
+  EXPECT_EQ(gpu::MemoryAllocation::CUTOFF_ALLOW_REQUIRED_ONLY,
+#else
   EXPECT_EQ(gpu::MemoryAllocation::CUTOFF_ALLOW_NICE_TO_HAVE,
+#endif
             policy.priority_cutoff_when_visible);
 }
 #endif