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This is part I of the effort to support third party authentication for android.

The TokenFetcherProxy is generalized version of the existing PepperTokenFetcher.  It is used by the ThirdPartyClientAuthenticator to fetch auth tokens from both the webapp and the android client.  

BUG=329109

Review URL: https://codereview.chromium.org/311983003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275386 0039d316-1c4b-4281-b951-d872f2087c98