Oilpan: Fix tracing error in CanvasRenderingContext2D

We need to trace CanvasRenderingContext in non-oilpan builds as well.
Otherwise the plugin complains in non-oilpan builds.

TBR=ager

Review URL: https://codereview.chromium.org/259103005

git-svn-id: svn://svn.chromium.org/blink/trunk@172856 bbb929c8-8fbe-4397-9dbb-9b2b20218538