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This change fixes a particular issue where the Javascript "onfocus()"
event doesn't fire when accessibility API's call focus() on a
specific node when the frame containing that node doesn't currently
have focus.

Blink's implementation of "onfocus()" in Document::SetFocusedElement has
a condition under GetPage()->GetFocusController().IsFocused() to prevent
focus from being fired if the page doesn't have focus. This is correct
behavior from Javascript's perspective.

From the accessibility side, behavior was already added to focus on the
document before focusing on a specific node in
https://chromium-review.googlesource.com/c/chromium/src/+/1809943

However, when that change calls into
RenderFrameHostImpl::AccessibilityViewSetFocus, if the view is a
RenderWidgetHostViewChildFrame, nothing will happen, as that method is
empty. So this change completes that method with a simple focus
implementation.

A browser test was added to confirm that the Javascript onfocus event
is properly fired.

Bug: 1124067
Change-Id: I5a77fcdfba96da45fe548e93c2da8c9a69181066
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2389005
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805465}