[AX Mac] implement accessibilityPerformDecrement/Increment

Implement accessibilityPerformDecrement/Increment methods, and hide
their NSAccessibilityDecrement/IncrementAction action counterparts
under a flag.

Based on crrev.com/c/5846568.

Test: `out/.../content_browsertests \
   --gtest_filter="*AccessibilityPerformIncrement*"`

Test: `out/.../content_browsertests \
   --gtest_filter="*AccessibilityPerformDecrement*"`

Include-Ci-Only-Tests: true
Cq-Include-Trybots: luci.chromium.try:mac-fieldtrial-tester

Bug: 363275809
Change-Id: Iff285d45a8518903b80db02ea9d23f87cc42d7b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6115619
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Jayson Adams <shrike@chromium.org>
Commit-Queue: Alexander Surkov <asurkov@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1406469}