Fix focus navigation for nested slot case

Fix focus navigation for nested slot case

For forward navigation, SlotScopedTraversal was not
excluding an element that is assigned to another slot.

For backward navigation, the original logic in
FocusController/SlotScopedTraversal failed to
detect case when shadow hosts exist in a slot scope.
Elements that are slotted to slots in their shadow roots
should be skipped in traversal.

BUG=618587
Review-Url: https://codereview.chromium.org/2432293002
Cr-Commit-Position: refs/heads/master@{#428682}
(cherry picked from commit 5639462246b2b792a1d75ba418fce6c6edd76940)

Committed: https://chromium.googlesource.com/chromium/src/+/bcc1bf393bb93e080c224e743a9bb972948db27e

[kochi, 2016-11-08 00:36:10]

Description was changed from

==========
Fix focus navigation for nested slot case

For forward navigation, SlotScopedTraversal was not
excluding an element that is assigned to another slot.

For backward navigation, the original logic in
FocusController/SlotScopedTraversal failed to
detect case when shadow hosts exist in a slot scope.
Elements that are slotted to slots in their shadow roots
should be skipped in traversal.

BUG=618587

Review-Url: https://codereview.chromium.org/2432293002
Cr-Commit-Position: refs/heads/master@{#428682}
(cherry picked from commit 5639462246b2b792a1d75ba418fce6c6edd76940)
==========

to

==========
Fix focus navigation for nested slot case

For forward navigation, SlotScopedTraversal was not
excluding an element that is assigned to another slot.

For backward navigation, the original logic in
FocusController/SlotScopedTraversal failed to
detect case when shadow hosts exist in a slot scope.
Elements that are slotted to slots in their shadow roots
should be skipped in traversal.

BUG=618587

Review-Url: https://codereview.chromium.org/2432293002
Cr-Commit-Position: refs/heads/master@{#428682}
(cherry picked from commit 5639462246b2b792a1d75ba418fce6c6edd76940)

Committed:
https://chromium.googlesource.com/chromium/src/+/bcc1bf393bb93e080c224e743a9b...
==========

[kochi, 2016-11-08 00:36:13]

Committed patchset #1 (id:1) manually as
bcc1bf393bb93e080c224e743a9bb972948db27e.