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Currently the shared code is located in desktop_viewport.js, which means
that this file needs to be included by the AppRemoting builds. But
including that file also brings in a bunch of DesktopRemoting stuff as
well.

This cl extracts out the shared code into viewport.js (and moves the
unittests as well) so that we can more easily exclude the desktop
code from the AppRemoting code in a following cl.

BUG=

Review URL: https://codereview.chromium.org/1109453005

Cr-Commit-Position: refs/heads/master@{#327053}