[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
Review URL: https://codereview.chromium.org/
1135933002
Cr-Commit-Position: refs/heads/master@{#329206}