[Media Router] Simplify WebUI element positioning functions.

[Media Router] Simplify WebUI element positioning functions.

Removed shownComponentsChanged_() in favor of updateElementPositioning_(). Moved async() method to ensure any microtasks such as first run flow or issue stamping has concluded before the positioning of the WebUI elements is determined.

BUG=583203
Review URL: https://codereview.chromium.org/1652203004

Cr-Commit-Position: refs/heads/master@{#373116}
(cherry picked from commit 75b93c431dd27c7069969e2d300312284ee886a1)

Committed: https://chromium.googlesource.com/chromium/src/+/c09143df20415e8730ce516fe060f37db48f7a8d

[apacible, 2016-02-09 01:04:54]

Description was changed from

==========
[Media Router] Simplify WebUI element positioning functions.

Removed shownComponentsChanged_() in favor of updateElementPositioning_(). Moved
async() method to ensure any microtasks such as first run flow or issue stamping
has concluded before the positioning of the WebUI elements is determined.

BUG=583203

Review URL: https://codereview.chromium.org/1652203004

Cr-Commit-Position: refs/heads/master@{#373116}
(cherry picked from commit 75b93c431dd27c7069969e2d300312284ee886a1)
==========

to

==========
[Media Router] Simplify WebUI element positioning functions.

Removed shownComponentsChanged_() in favor of updateElementPositioning_(). Moved
async() method to ensure any microtasks such as first run flow or issue stamping
has concluded before the positioning of the WebUI elements is determined.

BUG=583203

Review URL: https://codereview.chromium.org/1652203004

Cr-Commit-Position: refs/heads/master@{#373116}
(cherry picked from commit 75b93c431dd27c7069969e2d300312284ee886a1)

Committed:
https://chromium.googlesource.com/chromium/src/+/c09143df20415e8730ce516fe060...
==========

[apacible, 2016-02-09 01:04:55]

Committed patchset #1 (id:1) manually as
c09143df20415e8730ce516fe060f37db48f7a8d.