remoting/client/plugin contained some files that didn't depend on PPAPI
and had unittests. Once the plugin is removed from chrome the plugin
will only be compiled for PNaCl, but in order to keep the unittests
working these files will still need to be compiled natively. Moving them
out of the plugin directory makes it clear which files are compiled
only for PNaCl. Also the moved files are not specific to the plugin
and would be useful in a native app if we were to implement one.

BUG=510271

Review URL: https://codereview.chromium.org/1228333004

Cr-Commit-Position: refs/heads/master@{#338977}