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 83899a226b2264ff17ca291e8387f0e69267f284..d000f4d347ddc1bbd22c8e5494e360711ea087f5 100644
--- a/chrome/browser/tab_contents/thumbnail_generator.cc
+++ b/chrome/browser/tab_contents/thumbnail_generator.cc
@@ -21,6 +21,7 @@
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/render_view_host.h"
+#include "content/public/browser/render_widget_host_view.h"
 #include "content/public/browser/web_contents.h"
 #include "googleurl/src/gurl.h"
 #include "skia/ext/image_operations.h"
@@ -72,14 +73,25 @@ SkBitmap GetBitmapForRenderWidgetHost(
     ThumbnailGenerator::ClipResult* clip_result) {
   base::TimeTicks begin_compute_thumbnail = base::TimeTicks::Now();
 
-  SkBitmap result;
-
   // 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.
+  // fails. First try to read from the compositing surface, then to read from
+  // the backing store.
+  content::RenderWidgetHostView* view = render_widget_host->GetView();
   skia::PlatformCanvas temp_canvas;
-  if (!render_widget_host->CopyFromBackingStore(&temp_canvas))
-    return result;
+  bool copy_result = false;
+  // TODO(mazda): Copy the shrinked size of compositing surface instead of the
+  // whole view size.
+  if (view) {
+    copy_result = render_widget_host->CopyFromCompositingSurface(
+        view->GetViewBounds().size(),
+        &temp_canvas);
+  }
+  if (!copy_result)
+      copy_result = 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
@@ -91,6 +103,7 @@ 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(