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Problem:
Imagine the focused element F "scrolled out" = hidden outside
of its container C's viewable area. In this case spatnav would
start its search for focus candidates up at the document root
and therefor miss (possibly nearby) candidates in C.

Solution:
Let AdvanceFocusDirectionallyInContainer() use the currently
focused element F's container C as its starting rectangle unless
F is an HTMLAreaElement. For HTMLAreaElement the search starts in
the HTMLAreaElement's image.

Bug: 763206
Change-Id: I4b721e92bf47ff5361d6f0fa8b70805eb0b485c2
Reviewed-on: https://chromium-review.googlesource.com/826345Reviewed-by: Hugo Holgersson <hugoh@vewd.com>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524743}