Cross-process iframe accessibility.
This change completes the plumbing to join cross-process iframes into a single composed accessibility tree on platforms that implement native accessibility APIs (Windows, Mac, Android).
Further work will be needed to update some accessibility API implementations to be multi-frame-aware.
BUG=368298
Committed: https://crrev.com/
387942c041da17ea6337bc0a81e96619e67e4ac4
Cr-Commit-Position: refs/heads/master@{#294118}
Review URL: https://codereview.chromium.org/
268543008
Cr-Commit-Position: refs/heads/master@{#294210}