Extract Windows string_conversion target (flutter#26029)

String conversion between UTF-8 and UTF-16 encodings is useful even in
the absence of other common Windows functionality.

This is pre-factoring for uwptool, which adds an adb-like helper tool
for install, uninstall, and launch of UWP applications for Windows
platforms.

See: flutter#81756

Author: cbracken

shell/platform/windows/BUILD.gn

@@ -74,8 +74,6 @@ source_set("flutter_windows_source") {
     "keyboard_key_handler.h",
     "platform_handler.cc",
     "platform_handler.h",
-    "string_conversion.cc",
-    "string_conversion.h",
     "system_utils.h",
     "task_runner.h",
     "text_input_plugin.cc",
@@ -152,6 +150,8 @@ source_set("flutter_windows_source") {
     defines = [ "FLUTTER_ENGINE_NO_PROTOTYPES" ]
   }
 
+  public_deps = [ ":string_conversion" ]
+
   deps = [
     ":flutter_windows_headers",
     "//flutter/shell/platform/common:common_cpp",
@@ -171,6 +171,19 @@ source_set("flutter_windows_source") {
   ]
 }
 
+# String encoding conversion utilities.
+source_set("string_conversion") {
+  sources = [
+    "string_conversion.cc",
+    "string_conversion.h",
+  ]
+
+  if (target_os == "winuwp") {
+    configs += [ ":cppwinrt_defs" ]
+    cflags = [ "/EHsc" ]
+  }
+}
+
 copy("publish_headers_windows") {
   sources = _public_headers
   outputs = [ "$root_out_dir/{{source_file_part}}" ]