CanvasRenderingContext2D should update the render tree to avoid stale computed style.

Before accessing the computed style for populating 'font' property of CanvasRenderingContext2D
we have to make sure we have a up-to-date render tree to avoid any stale reference.

BUG=411058

Review URL: https://codereview.chromium.org/552053003

git-svn-id: svn://svn.chromium.org/blink/trunk@182137 bbb929c8-8fbe-4397-9dbb-9b2b20218538