The new behavior for the unified banner means that individual tabs fetch

and keep track of pending review/draft IDs more eagerly than before. The

result of this is that it was easier to hit bugs when performing actions

with a review request open in multiple tabs.
This change adds a small utility that makes use of the

BroadcastChannel API to coordinate across multiple tabs/windows within

a browser session. Right now the only message that we send is "reload",

which asks other tabs to reload their contents. This is sent when

something is either published or discarded, which invalidates draft IDs.
The broadcast channel is origin-wide, so each PageView can implement a

method to return a data blob that's used for filtering whether or not to

reload. The default implementation is empty, and ReviewablePageView

will filter based on the review request ID (so any open tabs related to

that review request will be triggered).

Ran js-tests.
Published review requests, reviews, and replies from one tab, and saw

  other tabs pop up the alert and reload.
Discarded drafts from the unified banner and from the review dialog,

  and saw other tabs pop up the alert and reload.