Fix a missing GraphicsContext save

R=eae@chromium.org, senorblanco@chromium.org
BUG=257352

Review URL: https://codereview.chromium.org/20186003

git-svn-id: svn://svn.chromium.org/blink/trunk@154917 bbb929c8-8fbe-4397-9dbb-9b2b20218538