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

cc/test/test_web_graphics_context_3d.cc

Index: cc/test/test_web_graphics_context_3d.cc
diff --git a/cc/test/test_web_graphics_context_3d.cc b/cc/test/test_web_graphics_context_3d.cc
index f5ebb6b65c4b0d7b769c75fe69206463593e8951..3ea937999052cdbf024d3f934eb496d03640a696 100644
--- a/cc/test/test_web_graphics_context_3d.cc
+++ b/cc/test/test_web_graphics_context_3d.cc
@@ -364,6 +364,12 @@ void TestWebGraphicsContext3D::signalSyncPoint(
   sync_point_callbacks_.push_back(callback);
 }
 
+void TestWebGraphicsContext3D::signalQuery(
+    WebKit::WebGLId query,
+    WebGraphicsSyncPointCallback* callback) {
+  sync_point_callbacks_.push_back(callback);
+}
+
 void TestWebGraphicsContext3D::setSwapBuffersCompleteCallbackCHROMIUM(
     WebGraphicsSwapBuffersCompleteCallbackCHROMIUM* callback) {
   if (support_swapbuffers_complete_callback_)