Enable RGBA_4444 in GpuRasterBufferProvider

cc/raster/gpu_raster_buffer_provider.cc

Index: cc/raster/gpu_raster_buffer_provider.cc
diff --git a/cc/raster/gpu_raster_buffer_provider.cc b/cc/raster/gpu_raster_buffer_provider.cc
index 50dc9fc951b1697f41cd1861df41e5cb69af1ede..1cc36f18b36a9cb0d5574c50a3f49c41d46e1367 100644
--- a/cc/raster/gpu_raster_buffer_provider.cc
+++ b/cc/raster/gpu_raster_buffer_provider.cc
@@ -120,12 +120,14 @@ GpuRasterBufferProvider::GpuRasterBufferProvider(
     ResourceProvider* resource_provider,
     bool use_distance_field_text,
     int gpu_rasterization_msaa_sample_count,
+    ResourceFormat preferred_tile_format,
     bool async_worker_context_enabled)
     : compositor_context_provider_(compositor_context_provider),
       worker_context_provider_(worker_context_provider),
       resource_provider_(resource_provider),
       use_distance_field_text_(use_distance_field_text),
       msaa_sample_count_(gpu_rasterization_msaa_sample_count),
+      preferred_tile_format_(preferred_tile_format),
       async_worker_context_enabled_(async_worker_context_enabled) {
   DCHECK(compositor_context_provider);
   DCHECK(worker_context_provider);
@@ -175,6 +177,13 @@ void GpuRasterBufferProvider::OrderingBarrier() {
 
 ResourceFormat GpuRasterBufferProvider::GetResourceFormat(
     bool must_support_alpha) const {

[ccameron, 2017/03/23 22:20:14]

It feels wrong that we now have 3 copy-paste instances of this same
preferred_tile_format.

So I'd sort of feel like bumping this bit up to the base class (and the
distinction between best_render_buffer_format v best_texture_format) ... except
the base class is pure virtual, and it may be that people prefer to keep it that
way.

[ericrk, 2017/03/24 01:46:32]

I think I'd prefer to keep the base pure virtual. If we wanted to factor stuff
out, we could put an intermediate class between the base class and these guys
(or add a static util function like PlaybackToMemory).

Not sure how to factor this out while maintaining the
IsRenderbufferSupported/IsTextureSupported distinction without another virtual
or tracking var, so it doesn't seem worth it in this case?

+  if (resource_provider_->IsRenderBufferFormatSupported(
+          preferred_tile_format_) &&
+      (DoesResourceFormatSupportAlpha(preferred_tile_format_) ||
+       !must_support_alpha)) {
+    return preferred_tile_format_;
+  }
+
   return resource_provider_->best_render_buffer_format();
 }