Remove [LegacyTreatAsPartialInterface] from CanvasPathMethods

Easy one.

TBR=haraken
BUG=360435

Review URL: https://codereview.chromium.org/224733012

git-svn-id: svn://svn.chromium.org/blink/trunk@170958 bbb929c8-8fbe-4397-9dbb-9b2b20218538