[Chromoting] Add GN build support for ar_sample_app target.

[Chromoting] Add GN build support for ar_sample_app target.

This adds GN build rules for the AppRemoting sample app.
Some DesktopRemoting build rules were prettified as part of this
process (to make the App- and Desktop- remoting build rules more
similar).

This re-lands crrev.com/1126393002 without any additional changes
required. The issues with the GN clobber builds are fixed in
crrev.com/1137753002.

BUG=471916
Committed: https://crrev.com/581797d8f76d5855b7f0bed1fe912a038d1d7d25
Cr-Commit-Position: refs/heads/master@{#329206}

[garykac, 2015-05-08 23:18:47]

garykac@chromium.org changed reviewers:
+ jamiewalch@chromium.org

[Jamie, 2015-05-08 23:38:04]

Rubber-stamp LGTM.

[garykac, 2015-05-11 16:45:14]

The CQ bit was checked by garykac@chromium.org

[commit-bot: I haz the power, 2015-05-11 16:46:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1135933002/1

[commit-bot: I haz the power, 2015-05-11 18:46:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-11 18:48:01]

Patchset 1 (id:??) landed as
https://crrev.com/581797d8f76d5855b7f0bed1fe912a038d1d7d25
Cr-Commit-Position: refs/heads/master@{#329206}

[msw, 2015-05-11 20:21:26]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1136673003/ by msw@chromium.org.

The reason for reverting is: Broke GN bots, eg.:
http://build.chromium.org/p/chromium.linux/builders/Linux%20GN/builds/28281.

[msw, 2015-05-11 20:29:30]

msw@chromium.org changed reviewers:
+ msw@chromium.org

[msw, 2015-05-11 20:29:31]

Dirk tracked this down to a merge race with
https://codereview.chromium.org/1133913002