tabstrip: Use a PaintRecorder to access the Canvas for painting.

ui/compositor/compositing_recorder.cc

 // Copyright 2015 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/compositor/compositing_recorder.h"
 
 #include "ui/compositor/paint_context.h"
 #include "ui/gfx/canvas.h"
 
 namespace ui {
 
 CompositingRecorder::CompositingRecorder(const PaintContext& context,
-                                         float opacity)
-    : canvas_(context.canvas()) {
-  canvas_->SaveLayerAlpha(0xff * opacity);
+                                         uint8_t alpha)
+    : canvas_(context.canvas()), saved_(alpha < 255) {
+  if (saved_)
+    canvas_->SaveLayerAlpha(alpha);
 }
 
 CompositingRecorder::~CompositingRecorder() {
-  canvas_->Restore();
+  if (saved_)
+    canvas_->Restore();
 }
 
 }  // namespace ui