Making SkCanvas object ownership more robust between ImageBuffer and Canvas2DLayerBridge

Currently, we use an OwnPtr and a dumb ptr for storing references to
SkCanvas objects. This makes canvas teardown code very fragile
because of object destruction order issues.  Now that it has been
established and accepted that WTF::RefPtr can be used with SkRefCnt,
let's use RefPtr to make this code more robust.

BUG=244427
TEST=Covered by existing Canvas2DLayerBridge unit test and canvas layout tests
R=senorblanco

Review URL: https://chromiumcodereview.appspot.com/23515006

git-svn-id: svn://svn.chromium.org/blink/trunk@157910 bbb929c8-8fbe-4397-9dbb-9b2b20218538