Make WebGLRenderingContext.getSupportedExtensions()'s return type nullable

This allows it to return null when the context is lost, without resorting
to a custom binding for the method.

BUG=390780

Review URL: https://codereview.chromium.org/377113003

git-svn-id: svn://svn.chromium.org/blink/trunk@177911 bbb929c8-8fbe-4397-9dbb-9b2b20218538