updating Input field value by script should reset IME composition.

third_party/WebKit/Source/core/editing/InputMethodController.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 540 matching lines @@
     return;
 
   // An open typing command that disagrees about current selection would cause
   // issues with typing later on.
   TypingCommand::CloseTyping(frame_);
 
   // No DOM update after 'compositionend'.
   DispatchCompositionEndEvent(GetFrame(), g_empty_string);
 }
 
+void InputMethodController::CancelCompositionIfSelectionIsInvalid() {
+  if (!has_composition_)
+    return;
+
+  /*if (composition_range_)
+    LOG(INFO) << composition_range_->toString();
+  else
+    LOG(INFO) << "null range";
+  if (!HasComposition() || GetEditor().PreventRevealSelection())
+         return;
+  
+       // Check if selection start and selection end are valid. 
+  FrameSelection& selection = GetFrame().Selection();
+     if (selection.IsAvailable() && !composition_range_->collapsed()) {
+       const SelectionInDOMTree& selection_in_dom = selection.GetSelectionInDOMTree();
+         if (selection_in_dom.Base().CompareTo(composition_range_->StartPosition()) >= 0 &&
+           selection_in_dom.Extent().CompareTo(composition_range_->EndPosition()) <= 0)
+             return;
+    
+  }*/
+  
+       Clear();
+    GetFrame().GetChromeClient().DidCancelCompositionOnSelectionChange();
+  
+}
+
 void InputMethodController::SetComposition(
     const String& text,
     const Vector<CompositionUnderline>& underlines,
     int selection_start,
     int selection_end) {
   Editor::RevealSelectionScope reveal_selection_scope(&GetEditor());
 
   // Updates styles before setting selection for composition to prevent
   // inserting the previous composition text into text nodes oddly.
   // See https://bugs.webkit.org/show_bug.cgi?id=46868
@@ skipping 659 matching lines @@
   FinishComposingText(kKeepSelection);
 }
 
 DEFINE_TRACE(InputMethodController) {
   visitor->Trace(frame_);
   visitor->Trace(composition_range_);
   SynchronousMutationObserver::Trace(visitor);
 }
 
 }  // namespace blink