Ignore should-scroll-on-main-thread if main frame is not scrollable

cc/layers/layer_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/layer_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "base/strings/stringprintf.h"
 #include "cc/animation/animation_registrar.h"
 #include "cc/animation/scrollbar_animation_controller.h"
@@ skipping 366 matching lines @@
   // Because of the way scroll delta is calculated with a delegate, this will
   // leave the total scroll offset unchanged on this layer regardless of
   // whether a delegate is being used.
   scroll_offset_ += sent_scroll_delta_;
   scroll_delta_ -= sent_scroll_delta_;
   sent_scroll_delta_ = gfx::Vector2d();
 }
 
 InputHandler::ScrollStatus LayerImpl::TryScroll(
     gfx::PointF screen_space_point,
-    InputHandler::ScrollInputType type) const {
-
-  if (should_scroll_on_main_thread()) {
+    InputHandler::ScrollInputType type,
+    bool main_frame_is_scrollable) const {
+  // The reasons forcing use to scroll on the main thread here are applicable
+  // only for main frame scrolling. If we cannot scroll the main frame, then
+  // we can disregard these reasons.
+  if (should_scroll_on_main_thread() && main_frame_is_scrollable) {
     TRACE_EVENT_INSTANT0("cc,impl-scroll",
         "LayerImpl::TryScroll: Failed ShouldScrollOnMainThread",
         TRACE_EVENT_SCOPE_THREAD);
     return InputHandler::ScrollOnMainThread;
   }
 
   if (!screen_space_transform().IsInvertible()) {
     TRACE_EVENT_INSTANT0("cc,impl-scroll",
         "LayerImpl::TryScroll: Ignored NonInvertibleTransform",
         TRACE_EVENT_SCOPE_THREAD);
@@ skipping 896 matching lines @@
 
 size_t LayerImpl::GPUMemoryUsageInBytes() const { return 0; }
 
 scoped_ptr<base::Value> LayerImpl::AsValue() const {
   scoped_ptr<base::DictionaryValue> state(new base::DictionaryValue());
   AsValueInto(state.get());
   return state.PassAs<base::Value>();
 }
 
 }  // namespace cc