Refactor clip path paint definition tests

This change refactors several components of clippath tests, replacing
a lot of duplicated code, removing unnecessary code, and adding the
capability of detecting scheduled animation updates, with the intention
of making clip path unit tests easier to write and better at validating
invariants.

In detail:
  - Mock clip path paint image generator is removed entirely. This was
made unnecessary last year as the paint image generator test-only
branches were removed.
  - Added mock chrome client, which tracks if an animation update was
scheduled. This is useful for validating that excess main frame updates
are not being triggered
  - Moved all CC clip path correctness checks into one place rather
than duplicating them. This makes it clearer what the invariants are

Change-Id: I57343d667dca776a599f49849b6d71047f5fd31d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6200897
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Claire Chambers <clchambers@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1412336}