Update stroke path to use rect returned from isRect (to fix trailing moveTo bug)

Update stroke path to use rect returned from isRect (to fix trailing moveTo bug)

This basically recreates what was done in:

https://codereview.chromium.org/16950021/ (add rect-output parameter to isRect, allowing us to return the correct bounds even if a rectagular path has a trailing moveTo) with the addition of GM representation

BUG=skia:247770

Review URL: https://codereview.chromium.org/834503002