Fixing leaked SkDeferredCanvas in Canvas2DLayerBridge

When using RefPtr with SkRefCnt subclasses: it works, but there are no
adoption guards, so we have to be careful.  This leak was caused by an
un-adopted reference.

BUG=303546
TEST=valgrind layout test virtual/gpu/fast/canvas/canvas-resize-reset.html

Review URL: https://codereview.chromium.org/25538005

git-svn-id: svn://svn.chromium.org/blink/trunk@158863 bbb929c8-8fbe-4397-9dbb-9b2b20218538