Unifying changes for Page Action migration

Centralizing some changes into new functions to simplify the migration
of page actions. This includes ToolbarProvider::GetPageActionView to
return the icon views if the page action isn't migrated. Similarly,
PageActionInteractiveMixin::WaitForPageActionButtonVisible is updated to
wait for the element identifier for the view to support the icon view.
This also make it easier to audit the existing code.

Change-Id: I6f8ca5a9605ed554a96cdef02e096c79cb2b609d
Low-Coverage-Reason: LARGE_SCALE_REFACTOR
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6695965
Reviewed-by: Kaan Alsan <alsan@chromium.org>
Commit-Queue: Eshwar Stalin <estalin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1486760}