Fix cobalt_browsertests not execute test code (#8623)

This CL fixes an issue where cobalt_browsertests were not executing
their test bodies on Starboard.

**Root Cause**: 
On Starboard (similar to Android and iOS), ShellMainDelegate::RunProcess
initializes the BrowserMainRunner but returns 0 early, skipping the call
to BrowserMainRunner::Run().
* The test body (ui_task) is normally executed within
BrowserMainLoop::InterceptMainMessageLoopRun.
* On standard platforms, InterceptMainMessageLoopRun is called during
Run().
* Since Run() is skipped on Starboard, the test body was never executed.

  **Fix**:
Updated BrowserMainLoop::CreateStartupTasks to include
BUILDFLAG(IS_STARBOARD).
* This explicitly schedules InterceptMainMessageLoopRun as a startup
task.
* This ensures the ui_task is executed synchronously during
BrowserMainRunner::Initialize(), matching the behavior required by
ShellMainDelegate for these platforms.

  **Additional Changes**:
* Added assertions to ContentMainRunnerImplBrowserTest.StartupSequence
in content_main_runner_impl_browsertest.cc to verify that FeatureList,
ThreadPoolInstance, and ContentClient are correctly initialized after
startup.

Issue: 433354983

Author: linxinan-yt

cobalt/testing/browser_tests/content_main_runner_impl_browsertest.cc

@@ -297,6 +297,9 @@ class ContentMainRunnerImplBrowserTest : public ContentBrowserTest {
 
 IN_PROC_BROWSER_TEST_F(ContentMainRunnerImplBrowserTest, StartupSequence) {
   // All of the work is done in SetUp().
+  EXPECT_TRUE(base::FeatureList::GetInstance());
+  EXPECT_TRUE(base::ThreadPoolInstance::Get());
+  EXPECT_TRUE(GetContentClientForTesting());
 }
 
 }  // namespace

content/browser/browser_main_loop.cc

@@ -895,7 +895,10 @@ void BrowserMainLoop::CreateStartupTasks() {
 // is entered and startup tasks are run asynchronously from it.
 // InterceptMainMessageLoopRun() thus needs to be forced instead of happening
 // from MainMessageLoopRun().
-#if BUILDFLAG(IS_ANDROID) || BUILDFLAG(IS_IOS)
+// Make the Starboard same behavior like Android but only in non-release
+// builds for cobalt_browsertests.
+#if BUILDFLAG(IS_ANDROID) || BUILDFLAG(IS_IOS) || \
+    (!defined(OFFICIAL_BUILD) && BUILDFLAG(IS_STARBOARD))
   StartupTask intercept_main_message_loop_run = base::BindOnce(
       [](BrowserMainLoop* self) {
         // Lambda to ignore the return value and always keep a clean exit code