DevTools: fix path to WebGL idl

This patch changes path to WebGL idl from the WebGLRenderingContext to
WebGLRenderingContextBase.

BUG=
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/179463002

git-svn-id: svn://svn.chromium.org/blink/trunk@167753 bbb929c8-8fbe-4397-9dbb-9b2b20218538