Functions that return non-const internal data should not be const

as it provides a loophole around const.

I've removed const from platform_canvas() and sk_canvas(), if
anyone tries to access these from a const instance of Canvas, it
won't compile.

Review URL: https://codereview.chromium.org/560463004

Cr-Commit-Position: refs/heads/master@{#295104}

ui/gfx/canvas.h

@@ -421,8 +421,8 @@ class GFX_EXPORT Canvas {
                        const Rect& display_rect,
                        int flags);
 
-  skia::PlatformCanvas* platform_canvas() const { return owned_canvas_.get(); }
-  SkCanvas* sk_canvas() const { return canvas_; }
+  skia::PlatformCanvas* platform_canvas() { return owned_canvas_.get(); }
+  SkCanvas* sk_canvas() { return canvas_; }
   float image_scale() const { return image_scale_; }
 
  private: