Don't store results of skia::GetBitmapContext()

We'd like to get rid of skia::GetBitmapContext(); as a small step
in that direction while we figure out how to handle all of its
side effects, this moves the only invocation of it in Chrome proper
to the same function where it's used, avoiding the need to store
a context on the (mac-specific) CanvasSkiaPaint object.

R=danakj@chromium.org
BUG=543755

Review URL: https://codereview.chromium.org/1848883005

Cr-Commit-Position: refs/heads/master@{#384653}

ui/gfx/canvas_paint_mac.h

@@ -43,7 +43,6 @@ class GFX_EXPORT CanvasSkiaPaint : public Canvas {
  private:
   void Init(bool opaque);
 
-  CGContext* context_;
   NSRect rectangle_;
   // See description above setter.
   bool composite_alpha_;

ui/gfx/canvas_paint_mac.mm

@@ -8,25 +8,25 @@
 namespace gfx {
 
 CanvasSkiaPaint::CanvasSkiaPaint(NSRect dirtyRect)
-    : context_(NULL),
-      rectangle_(dirtyRect),
+    : rectangle_(dirtyRect),
       composite_alpha_(false) {
   Init(true);
 }
 
 CanvasSkiaPaint::CanvasSkiaPaint(NSRect dirtyRect, bool opaque)
-    : context_(NULL),
-      rectangle_(dirtyRect),
+    : rectangle_(dirtyRect),
       composite_alpha_(false) {
   Init(opaque);
 }
 
 CanvasSkiaPaint::~CanvasSkiaPaint() {
   if (!is_empty()) {
-    sk_canvas()->restoreToCount(1);
+    SkCanvas* canvas = sk_canvas();
+    canvas->restoreToCount(1);
 
     // Blit the dirty rect to the current context.
-    CGImageRef image = CGBitmapContextCreateImage(context_);
+    CGImageRef image =
+        CGBitmapContextCreateImage(skia::GetBitmapContext(*canvas));
     CGRect dest_rect = NSRectToCGRect(rectangle_);
 
     CGContextRef destination_context =
@@ -67,8 +67,6 @@
     // surface.
   canvas->translate(-SkDoubleToScalar(NSMinX(rectangle_)),
                     -SkDoubleToScalar(NSMinY(rectangle_)));
-
-  context_ = skia::GetBitmapContext(*canvas);
 }
 
 }  // namespace skia