Clear the owner element's widget in Document::shutdown().

Clear the owner element's widget in Document::shutdown().

FrameView::dispose() doesn't guarantee that the owner's widget is cleared
and this could be problematic when it's overwritten in
LocalFrame::createView() a short time later.

BUG=673170
Review-Url: https://codereview.chromium.org/2563313002
Cr-Commit-Position: refs/heads/master@{#438977}
(cherry picked from commit 71b91a424a00dce5ff5ff4cf2e7b52fc70136f36)

Committed: https://chromium.googlesource.com/chromium/src/+/a7a51f270ce442c7292abb84e2e4941f1c9c6199

[dcheng, 2016-12-19 18:29:39]

Description was changed from

==========
Clear the owner element's widget in Document::shutdown().

FrameView::dispose() doesn't guarantee that the owner's widget is cleared
and this could be problematic when it's overwritten in
LocalFrame::createView() a short time later.

BUG=673170

Review-Url: https://codereview.chromium.org/2563313002
Cr-Commit-Position: refs/heads/master@{#438977}
(cherry picked from commit 71b91a424a00dce5ff5ff4cf2e7b52fc70136f36)
==========

to

==========
Clear the owner element's widget in Document::shutdown().

FrameView::dispose() doesn't guarantee that the owner's widget is cleared
and this could be problematic when it's overwritten in
LocalFrame::createView() a short time later.

BUG=673170

Review-Url: https://codereview.chromium.org/2563313002
Cr-Commit-Position: refs/heads/master@{#438977}
(cherry picked from commit 71b91a424a00dce5ff5ff4cf2e7b52fc70136f36)

Committed:
https://chromium.googlesource.com/chromium/src/+/a7a51f270ce442c7292abb84e2e4...
==========

[dcheng, 2016-12-19 18:29:41]

Committed patchset #1 (id:1) manually as
a7a51f270ce442c7292abb84e2e4941f1c9c6199 (tree was closed).