Mere "Avoid unchecked casts in UA shadow DOM." to M56

Mere "Avoid unchecked casts in UA shadow DOM." to M56

If we have a way to leak UA shadow DOM to web authors or Chrome extension authors,
these unchecked casts would be dangerous.
This CL introduces toFooTypeOrDie() helpers, and apply it to UA shadow DOM.

BUG=668970
Review-Url: https://codereview.chromium.org/2534873004
Cr-Commit-Position: refs/heads/master@{#434899}
(cherry picked from commit 2ce7841faa0fcfb4bff02b808745a9399a1c0603)

Committed: https://chromium.googlesource.com/chromium/src/+/0db497a39cbefc87a1c63ebfa6e5b26eda899da7

[tkent, 2016-12-05 00:14:10]

Description was changed from

==========
Mere "Avoid unchecked casts in UA shadow DOM." to M56

If we have a way to leak UA shadow DOM to web authors or Chrome extension
authors,
these unchecked casts would be dangerous.
This CL introduces toFooTypeOrDie() helpers, and apply it to UA shadow DOM.

BUG=668970

Review-Url: https://codereview.chromium.org/2534873004
Cr-Commit-Position: refs/heads/master@{#434899}
(cherry picked from commit 2ce7841faa0fcfb4bff02b808745a9399a1c0603)
==========

to

==========
Mere "Avoid unchecked casts in UA shadow DOM." to M56

If we have a way to leak UA shadow DOM to web authors or Chrome extension
authors,
these unchecked casts would be dangerous.
This CL introduces toFooTypeOrDie() helpers, and apply it to UA shadow DOM.

BUG=668970

Review-Url: https://codereview.chromium.org/2534873004
Cr-Commit-Position: refs/heads/master@{#434899}
(cherry picked from commit 2ce7841faa0fcfb4bff02b808745a9399a1c0603)

Committed:
https://chromium.googlesource.com/chromium/src/+/0db497a39cbefc87a1c63ebfa6e5...
==========

[tkent, 2016-12-05 00:14:11]

Committed patchset #1 (id:1) manually as
0db497a39cbefc87a1c63ebfa6e5b26eda899da7.