Support browser side thumbnailing for GPU composited pages on Windows.

chrome/browser/tab_contents/thumbnail_generator.cc

Index: chrome/browser/tab_contents/thumbnail_generator.cc
diff --git a/chrome/browser/tab_contents/thumbnail_generator.cc b/chrome/browser/tab_contents/thumbnail_generator.cc
index 76c63c8a7e084761aa30b5568db1f5713c247fa1..8623a6838500fbf8213d4a72f5d7945606871241 100644
--- a/chrome/browser/tab_contents/thumbnail_generator.cc
+++ b/chrome/browser/tab_contents/thumbnail_generator.cc
@@ -70,14 +70,24 @@ SkBitmap GetBitmapForRenderWidgetHost(
     ThumbnailGenerator::ClipResult* clip_result) {
   base::TimeTicks begin_compute_thumbnail = base::TimeTicks::Now();
 
-  SkBitmap result;
+  bool is_accelerated_compositing_active = false;
+  content::RenderWidgetHostView* view = render_widget_host->view();
+  if (view) {
+    RenderWidgetHostImpl* rwhi = RenderWidgetHostImpl::FromRWHV(view);
+    is_accelerated_compositing_active =
+        rwhi->is_accelerated_compositing_active();
+  }
 
   // Get the bitmap as a Skia object so we can resample it. This is a large
   // allocation and we can tolerate failure here, so give up if the allocation
   // fails.
   skia::PlatformCanvas temp_canvas;
-  if (!render_widget_host->CopyFromBackingStore(&temp_canvas))
-    return result;
+  const bool copy_result = is_accelerated_compositing_active ?
+      render_widget_host->CopyFromCompositingSurface(&temp_canvas) :
+      render_widget_host->CopyFromBackingStore(&temp_canvas);
+  if (!copy_result)
+    return SkBitmap();
+
   const SkBitmap& bmp_with_scrollbars =
       skia::GetTopDevice(temp_canvas)->accessBitmap(false);
   // Clip the edgemost 15 pixels as that will commonly hold a scrollbar, which
@@ -89,6 +99,7 @@ SkBitmap GetBitmapForRenderWidgetHost(
   SkBitmap bmp;
   bmp_with_scrollbars.extractSubset(&bmp, scrollbarless_rect);
 
+  SkBitmap result;
   // Check if a clipped thumbnail is requested.
   if (options & ThumbnailGenerator::kClippedThumbnail) {
     SkBitmap clipped_bitmap = ThumbnailGenerator::GetClippedBitmap(