[cros] Use system request context for OAuth1 access token verification. Also retry if OAuth fetcher is cancelled during verification.
[cros] Use system request context for OAuth1 access token verification.
Also retry if OAuth fetcher is cancelled during verification.

BUG=122972
TEST=Manual. Sign with SPDY proxy enabled. Sign out. Notice that OAuth token is valid i.e. you're not asked for OTP.

Review URL: http://codereview.chromium.org/10008079

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@132222 0039d316-1c4b-4281-b951-d872f2087c98

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