Add half-float IOSurface GpuMemoryBuffer support

cc/raster/raster_buffer_provider.cc

Index: cc/raster/raster_buffer_provider.cc
diff --git a/cc/raster/raster_buffer_provider.cc b/cc/raster/raster_buffer_provider.cc
index bb49ae01a2b6c4184751f98ba30a3077ff75097b..c5f803e577f4a84c35f06d04ca244811d49cdede 100644
--- a/cc/raster/raster_buffer_provider.cc
+++ b/cc/raster/raster_buffer_provider.cc
@@ -27,7 +27,6 @@ bool IsSupportedPlaybackToMemoryFormat(ResourceFormat format) {
     case RGBA_4444:
     case RGBA_8888:
     case BGRA_8888:
-    case RGBA_F16:

[Daniele Castagna, 2017/03/30 21:47:10]

I'm curious, why was this here in the first place?

[ccameron, 2017/03/30 23:07:03]

I had added it when I added RGBA_F16 as a ResourceFormat, thinking "well, F16
should be render-able by software".

I re-thought this in this patch for two reasons. First is that software raster
to F16 is really slow. Second is that we don't have all of the plumbing
necessary to enable it (we don't set up skia to draw F16, etc).

[Daniele Castagna, 2017/03/31 00:25:20]

I didn't even know that Skia could do F16, that's why I asked. I thought they
would raster only BGR. Thanks for the explanation.

     case ETC1:
       return true;
     case ALPHA_8:
@@ -35,6 +34,7 @@ bool IsSupportedPlaybackToMemoryFormat(ResourceFormat format) {
     case RGB_565:
     case RED_8:
     case LUMINANCE_F16:
+    case RGBA_F16:
       return false;
   }
   NOTREACHED();