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When advancing focus into an iframe, allow the frame to focus itself
after deciding which element will be focused. This prevents sending a
focus event to the previous element in that frame only to blur it
immediately. Non-oopifs simply fire a focus event to the new element,
the previously focused element in that frame received a blur event when
another frame was focused.

The test ensures a consistent state when mixing tab and clicking
navigation. In specific cases, focus would early out when the element to
advance to was previously focused, and never cleared when the mouse is
clicked in another frame.

BUG=702330
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2796533002
Cr-Commit-Position: refs/heads/master@{#463006}