By default a WebGL context should have stencil buffer off

BUG=335273
TEST=webkit layout tests, gpu bots/webgl conformance tests
R=bajones@chromium.org, kbr@chromium.org

Review URL: https://codereview.chromium.org/134153009

git-svn-id: svn://svn.chromium.org/blink/trunk@165555 bbb929c8-8fbe-4397-9dbb-9b2b20218538