Do not calculate composition bounds until IME requests.

content/renderer/render_frame_impl.cc

 // Copyright 2013 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_frame_impl.h"
 
 #include <map>
 #include <string>
 #include <utility>
 #include <vector>
@@ skipping 1281 matching lines @@
     return;
   GetRenderWidget()->UpdateSelectionBounds();
 }
 
 void RenderFrameImpl::PepperCancelComposition(
     PepperPluginInstanceImpl* instance) {
   if (instance != focused_pepper_plugin_)
     return;
   Send(new InputHostMsg_ImeCancelComposition(render_view_->GetRoutingID()));
 #if defined(OS_MACOSX) || defined(USE_AURA)
-  GetRenderWidget()->UpdateCompositionInfo(true);
+  GetRenderWidget()->UpdateCompositionInfo(true, false /* force update */);

[yosin_UTC9, 2016/07/06 02:02:01]

How about

const bool shouldUpdateRange =  true;
const bool kNoUpdate = false;
GetRenderWidget()->UpdateCompositionInfo(shouldUpdateRange, kNoUpdate);

[Seigo Nonaka, 2016/07/06 03:25:56]

Done.

 #endif
 }
 
 void RenderFrameImpl::PepperSelectionChanged(
     PepperPluginInstanceImpl* instance) {
   if (instance != focused_pepper_plugin_)
     return;
   SyncSelectionIfRequired();
 }
 
@@ skipping 5014 matching lines @@
   // event target. Potentially a Pepper plugin will receive the event.
   // In order to tell whether a plugin gets the last mouse event and which it
   // is, we set |pepper_last_mouse_event_target_| to null here. If a plugin gets
   // the event, it will notify us via DidReceiveMouseEvent() and set itself as
   // |pepper_last_mouse_event_target_|.
   pepper_last_mouse_event_target_ = nullptr;
 #endif
 }
 
 }  // namespace content