MacViews/a11y: Implement NativeViewAccessibilityMac.

Remove Mac-specific code in the base NativeViewAccessibility class and move it
to a platform-specific NativeViewAccessibilityMac.

BUG=396137

Review-Url: https://codereview.chromium.org/2704263002
Cr-Commit-Position: refs/heads/master@{#451876}
5 files changed