ui: Remove use of bitmaps when painting tab backgrounds.

ui/gfx/scoped_canvas.h

 // 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.
 
 #ifndef UI_GFX_SCOPED_CANVAS_H_
 #define UI_GFX_SCOPED_CANVAS_H_
 
 #include "base/macros.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/gfx_export.h"
 
 namespace gfx {
 
 // Saves the drawing state, and restores the state when going out of scope.
 class GFX_EXPORT ScopedCanvas {
  public:
   explicit ScopedCanvas(gfx::Canvas* canvas) : canvas_(canvas) {
     if (canvas_)
       canvas_->Save();
   }
   ~ScopedCanvas() {
     if (canvas_)
       canvas_->Restore();
   }
 
+  ScopedCanvas(ScopedCanvas&& o) {
+    canvas_ = o.canvas_;
+    o.canvas_ = nullptr;
+  }
+  ScopedCanvas& operator=(ScopedCanvas&& o) {
+    if (canvas_)
+      canvas_->Restore();
+    canvas_ = o.canvas_;
+    o.canvas_ = nullptr;

[danakj, 2017/04/05 22:01:37]

Oops, was missing the return here when I changed these from =default at the last
minute. I realized that would lead to 2 restore for 1 save. The code currently
only uses the constructor (to emplace into optional).

[danakj, 2017/04/05 22:07:01]

Alternatively i could keep passing a null instead of using optional, optional
just seemed more obvious when reading it, lmk.

+  }
+
  private:
   gfx::Canvas* canvas_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedCanvas);
 };
 
 // Saves the drawing state.  If |flip| is true, and the UI is in RTL layout,
 // applies a transform such that anything drawn inside the supplied width will
 // be flipped horizontally.
 class GFX_EXPORT ScopedRTLFlipCanvas {
  public:
   ScopedRTLFlipCanvas(gfx::Canvas* canvas, int width, bool flip = true);
   ~ScopedRTLFlipCanvas() {}
 
  private:
   ScopedCanvas canvas_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedRTLFlipCanvas);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_SCOPED_CANVAS_H_