Supports the invisible underline for native input fields.

ui/base/ime/composition_text.cc

 // Copyright (c) 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 "ui/base/ime/composition_text.h"
 
 namespace ui {
 
 CompositionText::CompositionText() {
+  underlines.push_back(CompositionUnderline());

[msw, 2015/03/26 20:22:48]

Why do we need this change? It seems odd to add this underline to an empty
CompositionText object if it's just needed to accomodate
TextfieldModel::SetCompositionText's behavior at line 570: "if
(composition.underlines.size() > 0 && composition.underlines[0].color != 0)".

If we just want the textfield's composition range to be underlined by default,
unless the underline is specifically set to transparent, perhaps change the
condition in TextfieldModel::SetCompositionText to "if
(composition.underlines.empty() || composition.underlines[0].color != 0)"

[Shu Chen, 2015/03/30 03:56:21]

It was to make the tests can pass.
I've just checked the latest patchset that the test can still pass without this
hack.
So I just reverted this change. Thanks for catching!

+  underlines[0].color = SK_ColorBLACK;
 }
 
 CompositionText::~CompositionText() {
 }
 
 void CompositionText::Clear() {
   text.clear();
   underlines.clear();
   selection = gfx::Range();
 }
 
 }  // namespace ui