Reduce texture uploads during scrollbar invalidations.

cc/caching_bitmap_canvas_layer_texture_updater.cc

Index: cc/caching_bitmap_canvas_layer_texture_updater.cc
diff --git a/cc/caching_bitmap_canvas_layer_texture_updater.cc b/cc/caching_bitmap_canvas_layer_texture_updater.cc
new file mode 100644
index 0000000000000000000000000000000000000000..55ef4e605d141d0bd8bf9d0b2698389edba1ca39
--- /dev/null
+++ b/cc/caching_bitmap_canvas_layer_texture_updater.cc
@@ -0,0 +1,50 @@
+// Copyright 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+
+#include "caching_bitmap_canvas_layer_texture_updater.h"
+
+#include "LayerPainterChromium.h"
+#include "base/hash.h"
+#include "skia/ext/platform_canvas.h"
+
+namespace cc {
+
+PassRefPtr<CachingBitmapCanvasLayerTextureUpdater>
+CachingBitmapCanvasLayerTextureUpdater::Create(
+    PassOwnPtr<LayerPainterChromium> painter) {
+  return adoptRef(new CachingBitmapCanvasLayerTextureUpdater(painter));
+}
+
+CachingBitmapCanvasLayerTextureUpdater::CachingBitmapCanvasLayerTextureUpdater(
+    PassOwnPtr<LayerPainterChromium> painter)
+    : BitmapCanvasLayerTextureUpdater(painter),
+      pixels_did_change_(false),
+      pixel_hash_(0) {
+}
+
+void CachingBitmapCanvasLayerTextureUpdater::prepareToUpdate(
+    const IntRect& content_rect,
+    const IntSize& tile_size,
+    float contents_width_scale,
+    float contents_height_scale,
+    IntRect& resulting_opaque_rect,
+    CCRenderingStats& stats) {
+  BitmapCanvasLayerTextureUpdater::prepareToUpdate(content_rect,
+                                                   tile_size,
+                                                   contents_width_scale,
+                                                   contents_height_scale,
+                                                   resulting_opaque_rect,
+                                                   stats);
+
+  const SkBitmap& new_bitmap = m_canvas->getDevice()->accessBitmap(false);
+  SkAutoLockPixels lock(new_bitmap);
+  uint32 new_pixel_hash = base::Hash(static_cast<const char*>(new_bitmap.getPixels()), new_bitmap.getSafeSize());
+
+  pixels_did_change_ = new_pixel_hash != pixel_hash_;
+  pixel_hash_ = new_pixel_hash;
+}
+
+} // namespace cc