Merge to M50: TalkBack needs to know supported HTML element types

Merge to M50: TalkBack needs to know supported HTML element types

TalkBack wants to access the element type navigation we added
in M50 (http://crbug.com/583095), but we need to export the set of
supported element types so it knows which ones it can use.

BUG=592901, 583095
Review URL: https://codereview.chromium.org/1767313002

Cr-Commit-Position: refs/heads/master@{#380255}
(cherry picked from commit 3cbcd14c48a113a4288f1ae70aca7a24b718317d)

Committed: https://chromium.googlesource.com/chromium/src/+/d740d0e7262f2531b6ac5e24d5f47d8b55b4d737

[dmazzoni, 2016-03-10 21:03:14]

Description was changed from

==========
Merge to M50: TalkBack needs to know supported HTML element types

TalkBack wants to access the element type navigation we added
in M50 (http://crbug.com/583095), but we need to export the set of
supported element types so it knows which ones it can use.

BUG=592901,583095

Review URL: https://codereview.chromium.org/1767313002

Cr-Commit-Position: refs/heads/master@{#380255}
(cherry picked from commit 3cbcd14c48a113a4288f1ae70aca7a24b718317d)
==========

to

==========
Merge to M50: TalkBack needs to know supported HTML element types

TalkBack wants to access the element type navigation we added
in M50 (http://crbug.com/583095), but we need to export the set of
supported element types so it knows which ones it can use.

BUG=592901,583095

Review URL: https://codereview.chromium.org/1767313002

Cr-Commit-Position: refs/heads/master@{#380255}
(cherry picked from commit 3cbcd14c48a113a4288f1ae70aca7a24b718317d)

Committed:
https://chromium.googlesource.com/chromium/src/+/d740d0e7262f2531b6ac5e24d5f4...
==========

[dmazzoni, 2016-03-10 21:03:17]

Committed patchset #1 (id:1) manually as
d740d0e7262f2531b6ac5e24d5f47d8b55b4d737.