Enable NVPR with command buffer

Set Chromium command-line flag --enable-gl-path-rendering
before creating command buffer context. This is needed for
the command buffer to expose NVPR at the moment.

BUG=skia:2992
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1538863002

Review URL: https://codereview.chromium.org/1538863002