[Extensions Cleanup] Remove EventPageTracker

This class was only ever implemented by ProcessManager, and had two
methods WakeEventPage() and IsEventPageSuspended().

IsEventPageSuspended() was only used in a single test file, and just
checks that there is no background host for an extension. This can be
trivially done by checking GetBackgroundHostForExtension() (which is
what the implementation did).

WakeEventPage() is called by numerous sites, but all of them do so by
accessing the ProcessManager. No sites accept or use the class as the
EventPageTracker, so having a separate class that the ProcessManager
inherits from is unnecessary.

Bug: None
Change-Id: Ie83d9b5eb83dcb458a29ab51a46ea33797203af4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6044582
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1387960}