Accept "webgl" as WebGL context id

Now that WebGL is enabled on Android by default, accept "webgl" in
addition to "experimental-webgl" as the context id that is passed to
canvas.getContext().

BUG=112928

Review URL: https://chromiumcodereview.appspot.com/21801002

git-svn-id: svn://svn.chromium.org/blink/trunk@155607 bbb929c8-8fbe-4397-9dbb-9b2b20218538