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When navigating in the frame, the "first responder" that has focus is
the view. So when trying to move from frame to web content, the first
responder does not get updated, which makes VO not respond to the focus
events being fired within web content.

This fix is so that when a focus event is fired for the web content,
set it as the first responder if it has not been set yet. This only gets
called by screen readers when navigating with scan mode (arrow keys).

This change has been verified in Mac with VO and in Windows with
NVDA and Narrator (IA2 and UIA).

AccessibilityActionBrowserTest.FocusAction already tests the main action
of BrowserAccessibilityManager::SetFocus.

Bug: 990014
Change-Id: Iaec036a14743c77b26f68ef10b0515aa7034cdf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809943Reviewed-by: Ian Prest <iapres@microsoft.com>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702063}