Implement accessibilityInsertionPointLineNumber for BrowserAccessibilityCocoa

* Implement[BrowserAccessibilityCocoa accessibilityInsertionPointLineNumber]
  based on the existing, deprecated support.
* Put the code from [BrowserAccessibilityCocoa insertionPointLineNumber]
  in [BrowserAccessibilityCocoa AXInsertionPointLineNumber]. The latter
  is the expected (deprecated) selector name.
* Make [AXPlatformNodeCocoa accessibilityInsertionPointLineNumber]
  independent of [AXPlatformNodeCocoa AXInsertionPointLineNumber]. This
  will make it easier to remove the deprecated API.

Note: This API is not being put behind the migration flag because there
is Views code which expects the new API to always be a supported
selector on text fields. See AXNativeWidgetMacTest.ProtectedTextfields.

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

Test: `out/.../accessibility_unittests \
   --gtest_filter="*AccessibilityInsertionPointLineNumber*"`

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

Bug: 363275809
Change-Id: Iaf945ea2c6c349a162d9ef26ce6431fa637a5370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6063168
Commit-Queue: Joanmarie Diggs <jdiggs@igalia.com>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Jayson Adams <shrike@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1393150}