Add compositor support for half-float RGBA buffers and textures

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 d42f9b67728ed0750939d224f4a05d1408cead6a..c2b8c386914d3f2ca034a88c69cfbfbead7f8bed 100644
--- a/cc/raster/gpu_raster_buffer_provider.cc
+++ b/cc/raster/gpu_raster_buffer_provider.cc
@@ -49,8 +49,10 @@ static void RasterizeSource(
   SkSurface* sk_surface = scoped_surface.sk_surface();
   // Allocating an SkSurface will fail after a lost context.  Pretend we
   // rasterized, as the contents of the resource don't matter anymore.
-  if (!sk_surface)
+  if (!sk_surface) {
+    DLOG(ERROR) << "Failed to allocate raster surface";
     return;
+  }
 
   // Playback
   gfx::Rect playback_rect = raster_full_rect;