Limit pixels that get cleared to repaint area.

Since we now keep the canvas around in the ContentLayerUpdater, we often
end up getting small invalidations that paint on a large canvas. In those
cases, instead of clearing the entire canvas to transparent, clear only the
portion that will be repainted.

BUG=313494

Review URL: https://codereview.chromium.org/61403003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233539 0039d316-1c4b-4281-b951-d872f2087c98

cc/resources/content_layer_updater.cc

@@ -62,12 +62,14 @@ 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");
+    canvas->drawColor(SK_ColorTRANSPARENT, SkXfermode::kSrc_Mode);
+  }
 
   gfx::RectF opaque_layer_rect;
   painter_->Paint(canvas, layer_rect, &opaque_layer_rect);