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The previous remoting webapp GN build files were not enabled and at
least 6 months out of date. This cl fixes the problem by bringing
the build rules up to date and enables the webapp build.

It also starts to move all the remoting-specific build rules into
the remoting dir rather than having them in the main build.gn file.

BUG=

Review URL: https://codereview.chromium.org/1082083005

Cr-Commit-Position: refs/heads/master@{#325551}