Android: Second attempt at freeing GLHelper ashmem

This tears down the GLHelperHolder in RenderWidgetHostViewAndroid
when all activities get stopped.

But it only does this if there are no more readback
requests in flight.

BUG=641962,636630

Review-Url: https://codereview.chromium.org/2383613005
Cr-Commit-Position: refs/heads/master@{#423045}

content/browser/renderer_host/render_widget_host_view_android.h

@@ -262,15 +262,6 @@ class CONTENT_EXPORT RenderWidgetHostViewAndroid
 
   void UpdateBackgroundColor(SkColor color);
 
-  // Called after async screenshot task completes. Scales and crops the result
-  // of the copy.
-  static void PrepareTextureCopyOutputResult(
-      const gfx::Size& dst_size_in_pixel,
-      SkColorType color_type,
-      const base::TimeTicks& start_time,
-      const ReadbackRequestCallback& callback,
-      std::unique_ptr<cc::CopyOutputResult> result);
-
   // DevTools ScreenCast support for Android WebView.
   void SynchronousCopyContents(const gfx::Rect& src_subrect_in_pixel,
                                const gfx::Size& dst_size_in_pixel,