[Chromoting] Enable remoting webapp main.html GN build.
This is a re-land of crrev.com/1082083005
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least 6 months out of date.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/1096783002
Cr-Commit-Position: refs/heads/master@{#325733}
9 files changed
