Handle fallback content case in FlatTreeTraversal::traverseSiblings()

Handle fallback content case in FlatTreeTraversal::traverseSiblings()

FlatTreeTraversal::nextSibling()/previousSibling() didn't handle
fallback content (nodes in <slot> where nothing is distributed).

This CL adds fallback content handling and a unit test case.

BUG=683776
Review-Url: https://codereview.chromium.org/2749033006
Cr-Commit-Position: refs/heads/master@{#457399}
(cherry picked from commit b1c7a1d78f6c79f18f1686014a81b86abfb38806)

Review-Url: https://codereview.chromium.org/2756083002 .
Cr-Commit-Position: refs/branch-heads/3029@{#253}
Cr-Branched-From: 939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}
Committed: https://chromium.googlesource.com/chromium/src/+/b8e8f038e206c970e47d608a8181ef8f281d5c17

[kochi, 2017-03-17 02:02:31]

Description was changed from

==========
Handle fallback content case in FlatTreeTraversal::traverseSiblings()

FlatTreeTraversal::nextSibling()/previousSibling() didn't handle
fallback content (nodes in <slot> where nothing is distributed).

This CL adds fallback content handling and a unit test case.

BUG=683776

Review-Url: https://codereview.chromium.org/2749033006
Cr-Commit-Position: refs/heads/master@{#457399}
(cherry picked from commit b1c7a1d78f6c79f18f1686014a81b86abfb38806)
==========

to

==========
Handle fallback content case in FlatTreeTraversal::traverseSiblings()

FlatTreeTraversal::nextSibling()/previousSibling() didn't handle
fallback content (nodes in <slot> where nothing is distributed).

This CL adds fallback content handling and a unit test case.

BUG=683776

Review-Url: https://codereview.chromium.org/2749033006
Cr-Commit-Position: refs/heads/master@{#457399}
(cherry picked from commit b1c7a1d78f6c79f18f1686014a81b86abfb38806)

Review-Url: https://codereview.chromium.org/2756083002 .
Cr-Commit-Position: refs/branch-heads/3029@{#253}
Cr-Branched-From:
939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}
Committed:
https://chromium.googlesource.com/chromium/src/+/b8e8f038e206c970e47d608a8181...
==========

[kochi, 2017-03-17 02:02:37]

Committed patchset #1 (id:1) manually as
b8e8f038e206c970e47d608a8181ef8f281d5c17.