Add a Finch feature for using ANGLE's Metal backend by default.

Bug: angleproject:2634
Change-Id: I1a9adc2cb13e3377dbdd2eaca9d5bd877358b063
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435328Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811376}

ui/gl/gl_surface_egl.cc

@@ -816,6 +816,11 @@ void GetEGLInitDisplays(bool supports_angle_d3d,
     AddInitDisplay(init_displays, ANGLE_OPENGLES);
   }
 
+  if (supports_angle_metal && use_angle_default &&
+      base::FeatureList::IsEnabled(features::kDefaultANGLEMetal)) {
+    AddInitDisplay(init_displays, ANGLE_METAL);
+  }
+
   if (supports_angle_d3d) {
     if (use_angle_default) {
       // Default mode for ANGLE - try D3D11, else try D3D9

ui/gl/gl_switches.cc

@@ -216,6 +216,10 @@ const base::Feature kDirectCompositionUseNV12DecodeSwapChain{
 const base::Feature kDefaultANGLEOpenGL{"DefaultANGLEOpenGL",
                                         base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Default to using ANGLE's Metal backend.
+const base::Feature kDefaultANGLEMetal{"DefaultANGLEMetal",
+                                       base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Track current program's shaders at glUseProgram() call for crash report
 // purpose. Only effective on Windows because the attached shaders may only
 // be reliably retrieved with ANGLE backend.

ui/gl/gl_switches.h

@@ -94,6 +94,7 @@ GL_EXPORT extern const base::Feature kDirectCompositionSoftwareOverlays;
 GL_EXPORT extern const base::Feature kDirectCompositionUnderlays;
 GL_EXPORT extern const base::Feature kDirectCompositionUseNV12DecodeSwapChain;
 GL_EXPORT extern const base::Feature kDefaultANGLEOpenGL;
+GL_EXPORT extern const base::Feature kDefaultANGLEMetal;
 extern const base::Feature kTrackCurrentShaders;
 
 }  // namespace features