[Extensions] Simplify ExtensionActionAPI::Observer

Simplify ExtensionActionAPI::Observer by:
- Removing OnPageActionsUpdated(). This was only used by
  ChromeExtensionTestNotificationObserver when waiting for a certain
  number of page actions to be visible, and this can be achieved with
  the OnExtensionActionUpdated() notification instead.
- Removing OnExtensionActionVisibilityChanged(). It is never
  overridden by any observers.

Bug: 893373
Change-Id: Idc2310d14e44aadd9adb84cb93a8b9c6dee80175
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1564808
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653876}
5 files changed