Start running chrome_ai_wpt_tests on Mac bots

This CL introduce a new testing step named "chrome_ai_wpt_tests" in the
optimization guide Mac bots (arm64, x64). We runs the test only on Mac
bots due to the very limited capacity for some of the linux/windows
machines.

To run only on Mac, this cl does followings:

1. In infra/config/targets/basic_suites.star:
  - Add a new generated_script `chrome_ai_wpt_tests`.
2. In infra/config/targets/tests.star:
  - Add a new isolated_script_test `chrome_ai_wpt_tests`.
3. In infra/config/targets/matrix_compound_suites.star:
  - Splits `optimization_guide_desktop_script_tests` which is shared
    with Mac bots (mac-arm64, mac-x64) and win-arm64 bots to
    `optimization_guide_mac_script_tests` and
    `optimization_guide_win_arm64_script_tests`.
  - Add `chrome_ai_wpt_tests_suite` in
    `optimization_guide_mac_script_tests`.
4. In testing/buildbot/waterfalls.pyl:
  - Use `optimization_guide_mac_script_tests` for Mac bots (mac-arm64,
    mac-x64).
  - Use `optimization_guide_win_arm64_script_tests` for win-arm64 bots.

Bug: 361192155
Change-Id: Ib66052d23c8f0ef6e3b227056909e9c66167e655
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5824117
Reviewed-by: Marco Georgaklis <mgeorgaklis@google.com>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349675}