Add PERSISTENT_MAP usage for GpuMemoryBuffers.

content/renderer/gpu/compositor_dependencies.h

Index: content/renderer/gpu/compositor_dependencies.h
diff --git a/content/renderer/gpu/compositor_dependencies.h b/content/renderer/gpu/compositor_dependencies.h
index 9973a65cdf2619704ab693e4a3100483de1e606f..b8b1f1c714a3d52e843a2210f581617fed7f732d 100644
--- a/content/renderer/gpu/compositor_dependencies.h
+++ b/content/renderer/gpu/compositor_dependencies.h
@@ -42,7 +42,8 @@ class CompositorDependencies {
   virtual bool IsElasticOverscrollEnabled() = 0;
   // Only valid in single threaded mode.
   virtual bool UseSingleThreadScheduler() = 0;
-  virtual uint32 GetImageTextureTarget() = 0;
+  virtual uint32 GetMapImageTextureTarget() = 0;
+  virtual uint32 GetPersistentMapImageTextureTarget() = 0;

[reveman, 2015/05/20 14:26:08]

Having a different function for each configuration doesn't work long term. It
would be a mess to add a function for each combination of format and usage. Can
we pass format and usage as parameters instead?

   virtual scoped_refptr<base::SingleThreadTaskRunner>
   GetCompositorMainThreadTaskRunner() = 0;
   // Returns null if the compositor is in single-threaded mode (ie. there is no