Limit pixels that get cleared to repaint area.

cc/resources/content_layer_updater.cc

Index: cc/resources/content_layer_updater.cc
diff --git a/cc/resources/content_layer_updater.cc b/cc/resources/content_layer_updater.cc
index 92cabf2c66da9175da7cde477688eb4e353224e1..4a0fbf2605ff5bd7c0ac33d0ab3e6ae883514b76 100644
--- a/cc/resources/content_layer_updater.cc
+++ b/cc/resources/content_layer_updater.cc
@@ -62,12 +62,24 @@ void ContentLayerUpdater::PaintContents(SkCanvas* canvas,
   SkRect layer_sk_rect = SkRect::MakeXYWH(
       layer_rect.x(), layer_rect.y(), layer_rect.width(), layer_rect.height());
 
+  canvas->clipRect(layer_sk_rect);
+
   // If the layer has opaque contents then there is no need to
   // clear the canvas before painting.
-  if (!layer_is_opaque_)
-    canvas->clear(SK_ColorTRANSPARENT);
-
-  canvas->clipRect(layer_sk_rect);
+  if (!layer_is_opaque_) {
+    TRACE_EVENT0("cc", "Clear");
+
+    // If the canvas is substantially larger than the area that will be painted
+    // then it's faster to clear only the pixels within the clipRect.
+    const float kClearFasterThanDrawRatio = 0.8f;
+    int canvasArea =

[enne (OOO), 2013/11/06 01:59:45]

canvasArea => canvas_area

+        canvas->getDevice()->width() * canvas->getDevice()->height();
+    int clipArea = layer_rect.width() * layer_rect.height();

[enne (OOO), 2013/11/06 01:59:45]

clipArea => clip_area

+    if (kClearFasterThanDrawRatio * canvasArea < clipArea)
+      canvas->clear(SK_ColorTRANSPARENT);
+    else
+      canvas->drawColor(SK_ColorTRANSPARENT, SkXfermode::kSrc_Mode);
+  }
 
   gfx::RectF opaque_layer_rect;
   painter_->Paint(canvas, layer_rect, &opaque_layer_rect);