Ensure visual review toolbar script has appropriate cache headers.

The following discussion from !28736 (merged) should be addressed:

• @mikegreiling started a discussion: (+2 comments)

  I see we're using a static filename here – presumably so that we can make things simple for users loading this script into their review app cross-origin. However this could interfere with cache invalidation whenever this script gets updated. Have we taken measures to ensure this is sent with a no-cache or low TTL header?