Replace BrowserActionsContainer for extension menu

A skeleton class (ExtensionsToolbarContainer) is replacing it in the
toolbar when this flag is active. This skeleton will eventually fulfill
responsibilities of the old container but permits removing a significant
amount of code.

This is a breaking change (popups don't work, pinning doesn't work) that
is split out from a larger change. Follow-up CLs will reimplement those
features.

Bug: chromium:943702
Change-Id: If15f6fa18b8dc42c8fd4520e5fb2bcfbcfc79837
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592005
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656806}
20 files changed