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

content/common/view_messages.h

 // 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.
 
 // IPC messages for page rendering.
 // Multiply-included message file, hence no include guard.
 
 #include "base/memory/shared_memory.h"
 #include "base/process/process.h"
 #include "base/strings/string16.h"
@@ skipping 1272 matching lines @@
 // process to release the magnified image.
 IPC_MESSAGE_ROUTED1(ViewMsg_ReleaseDisambiguationPopupDIB,
                     TransportDIB::Handle /* DIB handle */)
 
 // Notifies the renderer that a snapshot has been retrieved.
 IPC_MESSAGE_ROUTED3(ViewMsg_WindowSnapshotCompleted,
                     int /* snapshot_id */,
                     gfx::Size /* size */,
                     std::vector<unsigned char> /* png */)
 
+#if defined(OS_MACOSX)
+// Notification of a change in scrollbar appearance and/or behavior.
+IPC_MESSAGE_CONTROL4(ViewMsg_UpdateScrollbarTheme,
+                     float /* initial_button_delay */,
+                     float /* autoscroll_button_delay */,
+                     bool /* jump_on_track_click */,
+                     bool /* redraw */)
+#endif
+
+#if defined(OS_ANDROID)
 // Tells the renderer to suspend/resume the webkit timers.
 IPC_MESSAGE_CONTROL1(ViewMsg_SetWebKitSharedTimersSuspended,
                      bool /* suspend */)
 
-#if defined(OS_ANDROID)
 // Sent when the browser wants the bounding boxes of the current find matches.
 //
 // If match rects are already cached on the browser side, |current_version|
 // should be the version number from the ViewHostMsg_FindMatchRects_Reply
 // they came in, so the renderer can tell if it needs to send updated rects.
 // Otherwise just pass -1 to always receive the list of rects.
 //
 // There must be an active search string (it is probably most useful to call
 // this immediately after a ViewHostMsg_Find_Reply message arrives with
 // final_update set to true).
@@ skipping 1062 matching lines @@
 // synchronously (see crbug.com/120597). This IPC message sends the character
 // bounds after every composition change to always have correct bound info.
 IPC_MESSAGE_ROUTED2(ViewHostMsg_ImeCompositionRangeChanged,
                     gfx::Range /* composition range */,
                     std::vector<gfx::Rect> /* character bounds */)
 #endif
 
 // Adding a new message? Stick to the sort order above: first platform
 // independent ViewMsg, then ifdefs for platform specific ViewMsg, then platform
 // independent ViewHostMsg, then ifdefs for platform specific ViewHostMsg.