Optimize alpha software PictureDrawQuads.

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index 61d0eaaafc4bfb63f0307f8961a933a2322093fc..6ca5bd2a7f654d8f1fe4819d2aaa277572b4effb 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -37,6 +37,7 @@
 #include "cc/trees/proxy.h"
 #include "cc/trees/single_thread_proxy.h"
 #include "gpu/GLES2/gl2extchromium.h"
+#include "skia/ext/alpha_proxy_canvas.h"
 #include "third_party/WebKit/public/platform/WebGraphicsContext3D.h"
 #include "third_party/khronos/GLES2/gl2.h"
 #include "third_party/khronos/GLES2/gl2ext.h"
@@ -1641,8 +1642,6 @@ void GLRenderer::DrawPictureQuadDirectToBackbuffer(
     const DrawingFrame* frame,
     const PictureDrawQuad* quad) {
   DCHECK(CanUseSkiaGPUBackend());
-  DCHECK_EQ(quad->opacity(), 1.f) << "Need to composite to a bitmap or a "
-                                     "render surface for non-1 opacity quads";
 
   // TODO(enne): This should be done more lazily / efficiently.
   gr_context_->resetContext();
@@ -1671,8 +1670,9 @@ void GLRenderer::DrawPictureQuadDirectToBackbuffer(
                                     &sk_device_matrix);
   sk_canvas_->setMatrix(sk_device_matrix);
 
+  skia::AlphaProxyCanvas proxy_canvas(sk_canvas_.get(), quad->opacity(), true);
   quad->picture_pile->RasterDirect(
-      sk_canvas_.get(), quad->content_rect, quad->contents_scale, NULL);
+      &proxy_canvas, quad->content_rect, quad->contents_scale, NULL);
 
   // Flush any drawing buffers that have been deferred.
   sk_canvas_->flush();