[Toolbar Customization] Categorize actions.

The handler organizes the actions into categories, and provides it to
the UI; the UI presents the actions per category.

Also adds unit tests for CustomizeToolbarHandler. This necessitated some
changes to PinnedToolbarActionsModel and the handler to make it unit
testable. Not adding tests yet for toolbar.ts and toolbar.html.ts to
help keep this CL to a manageable size.

Low-Coverage-Reason: TESTS_IN_SEPARATE_CL
Bug: 339301279
Change-Id: I4a45cdba55da73c3b8abbb8a4975e05adc59dc60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5545214


Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Chris Bookholt <bookholt@chromium.org>
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313042}