Make Windows accessibility event firing aware of guest / child frames.
Windows-specific fixes for accessibility of out-of-process iframes
and guest frames (e.g. <webview>):
1. When firing an event or doing a bounds calculation, we need to
call functions on the root BrowserAccessibilityManager's delegate
(not on the child frame's delegate).
2. When a MSAA client tries to look up an MSAA object by child ID,
we need to search child frames as well.
BUG=330307, 368298
Review URL: https://codereview.chromium.org/
799633007
Cr-Commit-Position: refs/heads/master@{#312080}