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This CL makes FtlClient use OAuthTokenGetter to retrieve access token
and attach it to the gRPC request. It also has some other changes in the
FTL playground:
* Move FtlSignalingPlayground into a separate class to make it cleaner.
* Make FtlSignalingPlayground prompt and read access code from the user
  and use it to start up an OAuthTokenGetter.

Bug: 927962
Change-Id: I43d2f19b53b41e2fa3652e6c6e4c09eb54ae889e
Reviewed-on: https://chromium-review.googlesource.com/c/1496317Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636861}