Do not give GPU memory to backgrounded compositors.

cc/output/gl_renderer.h

Index: cc/output/gl_renderer.h
diff --git a/cc/output/gl_renderer.h b/cc/output/gl_renderer.h
index f4885178a7d32ad62ddf2923b6b589da0f2e82d4..d65e6381723cd1b86a146a5ef6078d95f7fa6d3b 100644
--- a/cc/output/gl_renderer.h
+++ b/cc/output/gl_renderer.h
@@ -73,8 +73,6 @@ class CC_EXPORT GLRenderer : public DirectRenderer {
                                       size_t bytes_visible_and_nearby,
                                       size_t bytes_allocated) OVERRIDE;
 
-  virtual void SetDiscardBackBufferWhenNotVisible(bool discard) OVERRIDE;
-
   static void DebugGLCall(WebKit::WebGraphicsContext3D* context,
                           const char* command,
                           const char* file,
@@ -438,7 +436,6 @@ class CC_EXPORT GLRenderer : public DirectRenderer {
   gfx::Rect scissor_rect_;
   gfx::Rect viewport_;
   bool is_backbuffer_discarded_;
-  bool discard_backbuffer_when_not_visible_;
   bool is_using_bind_uniform_;
   bool visible_;
   bool is_scissor_enabled_;