[AppRemoting] Break out AppRemoting shared module.

This is a reland of crrev.com/1179873005 with the following fixes:
  Added GYP build of shared module
  Updated build-webapp.py script to not check env for shared module
    This fixes the GN Release build problem

This cl breaks out the shared AppRemoting code into a shared module so
that it does not need to be duplicated in each individual app.

This version has the following limitations which will be addressed in
follow-up cls:
(1) The apps have a hard-coded reference to the dev shared module
(2) The apps have a hard-coded reference to the dev orchestrator
(3) The resources in the app stub should be replaced with empty stub
    resources rather than a copy of all the shared resources.

BUG=

Review URL: https://codereview.chromium.org/1188253009

Cr-Commit-Position: refs/heads/master@{#337487}

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