Fix GraphicsContext::beginMaskedLayer() xferMode leak.

BUG=290552
R=reed@google.com,senorblanco@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24140002

git-svn-id: svn://svn.chromium.org/blink/trunk@157757 bbb929c8-8fbe-4397-9dbb-9b2b20218538