Perform glReadPixels with PBOs in the gpu, if PBOs are available.

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 ad6d3428de8f18c670386d80224c902fc696508f..ad013650468802f884df33f9fb06510abb2bfa9b 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -2301,6 +2301,14 @@ void GLRenderer::DoGetFramebufferPixels(
                                      NULL,
                                      GL_STREAM_READ));
 
+  WebKit::WebGLId query = 0;
+  if (is_async) {
+    query = context_->createQueryEXT();
+    GLC(context_, context_->beginQueryEXT(
+        GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM,
+        query));
+  }
+
   GLC(context_,
       context_->readPixels(window_rect.x(),
                            window_rect.y(),
@@ -2326,6 +2334,7 @@ void GLRenderer::DoGetFramebufferPixels(
                  base::Unretained(this),
                  cleanup_callback,
                  buffer,
+                 query,
                  dest_pixels,
                  window_rect.size());
   // Save the finished_callback so it can be cancelled.
@@ -2336,10 +2345,11 @@ void GLRenderer::DoGetFramebufferPixels(
   pending_async_read_pixels_.front()->buffer = buffer;
 
   if (is_async) {
-    unsigned sync_point = context_->insertSyncPoint();
-    SyncPointHelper::SignalSyncPoint(
+    GLC(context_, context_->endQueryEXT(
+        GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM));
+    SyncPointHelper::SignalQuery(
         context_,
-        sync_point,
+        query,
         finished_callback);
   } else {
     resource_provider_->Finish();
@@ -2352,10 +2362,15 @@ void GLRenderer::DoGetFramebufferPixels(
 void GLRenderer::FinishedReadback(
     const AsyncGetFramebufferPixelsCleanupCallback& cleanup_callback,
     unsigned source_buffer,
+    unsigned query,
     uint8* dest_pixels,
     gfx::Size size) {
   DCHECK(!pending_async_read_pixels_.empty());
 
+  if (query != 0) {
+    GLC(context_, context_->deleteQueryEXT(query));
+  }
+
   PendingAsyncReadPixels* current_read = pending_async_read_pixels_.back();
   // Make sure we service the readbacks in order.
   DCHECK_EQ(source_buffer, current_read->buffer);