Paint Profiler: erase bitmap before replaying SkPicture

We used to have garbage in resulting bitmap if we abort redraw before fill commands
in SkPicture are executed.

BUG=426100,426102

Review URL: https://codereview.chromium.org/654843010

git-svn-id: svn://svn.chromium.org/blink/trunk@184363 bbb929c8-8fbe-4397-9dbb-9b2b20218538