[bedrock] Address dangling refs in ExtensionsToolbarButton

Currently ExtensionsToolbarButton can host an anchored extension
menu bubble widget, which is opened / closed via
ExtensionsToolbarButton::ToggleExtensionsMenu().

However this method will show the Widget synchronously but close it
asynchronously. If closed while the toolbar and its Browser remains
alive this is fine, however it can be the case that if the bubble
is closed async as the Browser and toolbar are destroyed sync there
is a risk of dangling pointers.

This CL mitigates this risk by synchronously closing the extensions
menu if open when the button is destroyed. It also cleans up the
manually managed observation with a ScopedObservation.

Bug: 413168662
Change-Id: If52de8a577502a2e11b8ec5ed3c0a8cd62c3beb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6853423
Commit-Queue: Tom Lukaszewicz <tluk@chromium.org>
Reviewed-by: Fred Shih <ffred@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1502483}