[Media Router WebUI] Handle on dialog load focus on Mac.

[Media Router WebUI] Handle on dialog load focus on Mac.

This change better handles the non-user-triggered focus event that happens when the dialog loads. The behavior only appears on Mac and when the dialog initially is opened; however, it appears 100% of the time in these cases.

This adds an element with the sole purpose of triggering focus on that element when the dialog is open rather than any of the other header elements. This prevents messing with the tabindex (whether elements are focusable and in what priority) of the other elements. The tabindex of this element is then updated to be unfocusable so that the user cannot accidentally tab to the element.

BUG=594951
Review URL: https://codereview.chromium.org/1807803002

Cr-Commit-Position: refs/heads/master@{#381851}
(cherry picked from commit 48d7378cc2e2cc76c9cf0bfcd05413f8678662cb)

Committed: https://chromium.googlesource.com/chromium/src/+/49bc338d56358c724d7a0b3a45d4573e43e0b6f3

[apacible, 2016-03-21 18:14:17]

Description was changed from

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[Media Router WebUI] Handle on dialog load focus on Mac.

This change better handles the non-user-triggered focus event that happens when
the dialog loads. The behavior only appears on Mac and when the dialog initially
is opened; however, it appears 100% of the time in these cases.

This adds an element with the sole purpose of triggering focus on that element
when the dialog is open rather than any of the other header elements. This
prevents messing with the tabindex (whether elements are focusable and in what
priority) of the other elements. The tabindex of this element is then updated to
be unfocusable so that the user cannot accidentally tab to the element.

BUG=594951

Review URL: https://codereview.chromium.org/1807803002

Cr-Commit-Position: refs/heads/master@{#381851}
(cherry picked from commit 48d7378cc2e2cc76c9cf0bfcd05413f8678662cb)
==========

to

==========
[Media Router WebUI] Handle on dialog load focus on Mac.

This change better handles the non-user-triggered focus event that happens when
the dialog loads. The behavior only appears on Mac and when the dialog initially
is opened; however, it appears 100% of the time in these cases.

This adds an element with the sole purpose of triggering focus on that element
when the dialog is open rather than any of the other header elements. This
prevents messing with the tabindex (whether elements are focusable and in what
priority) of the other elements. The tabindex of this element is then updated to
be unfocusable so that the user cannot accidentally tab to the element.

BUG=594951

Review URL: https://codereview.chromium.org/1807803002

Cr-Commit-Position: refs/heads/master@{#381851}
(cherry picked from commit 48d7378cc2e2cc76c9cf0bfcd05413f8678662cb)

Committed:
https://chromium.googlesource.com/chromium/src/+/49bc338d56358c724d7a0b3a45d4...
==========

[apacible, 2016-03-21 18:14:20]

Committed patchset #1 (id:1) manually as
49bc338d56358c724d7a0b3a45d4573e43e0b6f3.