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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-re-lands crrev.com/1126393002 without any additional changes
required. The original issues with the GN clobber builds are fixed in
crrev.com/1137753002. The 2nd attempt to land was rejected due to
a merge race (fixed in this cl).

BUG=471916

Review URL: https://codereview.chromium.org/1143643003

Cr-Commit-Position: refs/heads/master@{#330243}