[Extensions] Allow overriding a nested options page for opening a new instance

[Extensions] Allow overriding a nested options page for opening a new instance

If there is an existing chrome://extensions page open, it should be able to be
overridden to show an extension's options page if that options page opens in
a view in the chrome://extensions page.

BUG=595253
Review URL: https://codereview.chromium.org/1805013005

Cr-Commit-Position: refs/heads/master@{#381555}
(cherry picked from commit 6ef722eae199b14ba8b6670568647b0905099442)

Committed: https://chromium.googlesource.com/chromium/src/+/3250f0efded0140f6c26ee6c7e63ddac0e2cc4a9

[Devlin, 2016-03-18 23:12:27]

Description was changed from

==========
[Extensions] Allow overriding a nested options page for opening a new instance

If there is an existing chrome://extensions page open, it should be able to be
overridden to show an extension's options page if that options page opens in
a view in the chrome://extensions page.

BUG=595253

Review URL: https://codereview.chromium.org/1805013005

Cr-Commit-Position: refs/heads/master@{#381555}
(cherry picked from commit 6ef722eae199b14ba8b6670568647b0905099442)
==========

to

==========
[Extensions] Allow overriding a nested options page for opening a new instance

If there is an existing chrome://extensions page open, it should be able to be
overridden to show an extension's options page if that options page opens in
a view in the chrome://extensions page.

BUG=595253

Review URL: https://codereview.chromium.org/1805013005

Cr-Commit-Position: refs/heads/master@{#381555}
(cherry picked from commit 6ef722eae199b14ba8b6670568647b0905099442)

Committed:
https://chromium.googlesource.com/chromium/src/+/3250f0efded0140f6c26ee6c7e63...
==========

[Devlin, 2016-03-18 23:12:28]

Committed patchset #1 (id:1) manually as
3250f0efded0140f6c26ee6c7e63ddac0e2cc4a9.