Have Blink be told of scrollbar changes rather than paying attention itself (Chromium side).

content/renderer/render_thread_impl.cc

 // Copyright (c) 2012 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 "content/renderer/render_thread_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <map>
 #include <vector>
@@ skipping 104 matching lines @@
 #if defined(OS_WIN)
 #include <windows.h>
 #include <objbase.h>
 #include "base/win/scoped_com_initializer.h"
 #else
 // TODO(port)
 #include "base/memory/scoped_handle.h"
 #include "content/child/npapi/np_channel_base.h"
 #endif
 
+#if defined(OS_MACOSX)
+#include "third_party/WebKit/public/web/mac/WebScrollbarTheme.h"
+#endif
+
 #if defined(OS_POSIX)
 #include "ipc/ipc_channel_posix.h"
 #endif
 
 #if defined(OS_ANDROID)
 #include <cpu-features.h>
 #include "content/renderer/android/synchronous_compositor_factory.h"
 #include "content/renderer/media/android/renderer_demuxer_android.h"
 #endif
 
@@ skipping 996 matching lines @@
   IPC_BEGIN_MESSAGE_MAP(RenderThreadImpl, msg)
     IPC_MESSAGE_HANDLER(ViewMsg_SetZoomLevelForCurrentURL,
                         OnSetZoomLevelForCurrentURL)
     // TODO(port): removed from render_messages_internal.h;
     // is there a new non-windows message I should add here?
     IPC_MESSAGE_HANDLER(ViewMsg_New, OnCreateNewView)
     IPC_MESSAGE_HANDLER(ViewMsg_PurgePluginListCache, OnPurgePluginListCache)
     IPC_MESSAGE_HANDLER(ViewMsg_NetworkStateChanged, OnNetworkStateChanged)
     IPC_MESSAGE_HANDLER(ViewMsg_TempCrashWithData, OnTempCrashWithData)
     IPC_MESSAGE_HANDLER(ViewMsg_SetRendererProcessID, OnSetRendererProcessID)
+#if defined(OS_ANDROID)
     IPC_MESSAGE_HANDLER(ViewMsg_SetWebKitSharedTimersSuspended,

[jam, 2013/11/06 22:34:43]

can you move the definition of this IPC to OS_ANDROID section as well? 

[Avi (use Gerrit), 2013/11/07 15:47:52]

Done.

                         OnSetWebKitSharedTimersSuspended)
+#endif
+#if defined(OS_MACOSX)
+    IPC_MESSAGE_HANDLER(ViewMsg_UpdateScrollbarTheme, OnUpdateScrollbarTheme)
+#endif
     IPC_MESSAGE_UNHANDLED(handled = false)
   IPC_END_MESSAGE_MAP()
   return handled;
 }
 
 void RenderThreadImpl::OnCreateNewView(const ViewMsg_New_Params& params) {
   EnsureWebKitInitialized();
   // When bringing in render_view, also bring in webkit's glue and jsbindings.
   RenderViewImpl::Create(
       params.opener_route_id,
@@ skipping 119 matching lines @@
 
 void RenderThreadImpl::OnTempCrashWithData(const GURL& data) {
   GetContentClient()->SetActiveURL(data);
   CHECK(false);
 }
 
 void RenderThreadImpl::OnSetRendererProcessID(base::ProcessId process_id) {
   renderer_process_id_ = process_id;
 }
 
+#if defined(OS_ANDROID)
 void RenderThreadImpl::OnSetWebKitSharedTimersSuspended(bool suspend) {
   ToggleWebKitSharedTimer(suspend);
 }
+#endif
+
+#if defined(OS_MACOSX)
+void RenderThreadImpl::OnUpdateScrollbarTheme(float initial_button_delay,
+                                              float autoscroll_button_delay,
+                                              bool jump_on_track_click,
+                                              bool redraw) {
+  WebKit::WebScrollbarTheme::updateScrollbars(initial_button_delay,
+                                              autoscroll_button_delay,
+                                              jump_on_track_click,
+                                              redraw);
+}
+#endif
 
 void RenderThreadImpl::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
   base::allocator::ReleaseFreeMemory();
 
   if (memory_pressure_level ==
       base::MemoryPressureListener::MEMORY_PRESSURE_CRITICAL) {
     // Trigger full v8 garbage collection on critical memory notification.
     v8::V8::LowMemoryNotification();
     // Clear the image cache.
@@ skipping 46 matching lines @@
   if (!gamepad_shared_memory_reader_)
     gamepad_shared_memory_reader_.reset(new GamepadSharedMemoryReader);
   gamepad_shared_memory_reader_->SampleGamepads(*data);
 }
 
 base::ProcessId RenderThreadImpl::renderer_process_id() const {
   return renderer_process_id_;
 }
 
 }  // namespace content