Use Sync FakeServer in exponential backoff tests

Use Sync FakeServer in exponential backoff tests

For SyncExponentialBackoffTest.OfflineToOnline, methods were added to
simulate and reset a network failure. The old (Python server) way of
doing this has been deleted from SyncTest.

For SyncExponentialBackoffTest.TransientErrorTest, a TRANSIENT_ERROR
is now triggered on FakeServer. The old way of doing this,
SyncTest.TriggerTransientError(), has been deleted.

BUG=406545
Committed: https://crrev.com/b1d8faa07507442e41c8a8a39217bcd2311b07fa
Cr-Commit-Position: refs/heads/master@{#299034}

[pval...(no longer on Chromium), 2014-10-08 22:25:31]

Patchset #1 (id:1) has been deleted

[pval...(no longer on Chromium), 2014-10-08 23:13:03]

Patchset #1 (id:20001) has been deleted

[pval...(no longer on Chromium), 2014-10-08 23:14:02]

pvalenzuela@chromium.org changed reviewers:
+ pavely@chromium.org

[pavely, 2014-10-09 01:01:06]

lgtm

[pval...(no longer on Chromium), 2014-10-09 23:32:39]

The CQ bit was checked by pvalenzuela@chromium.org

[commit-bot: I haz the power, 2014-10-09 23:33:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/642613002/40001

[commit-bot: I haz the power, 2014-10-10 01:37:56]

Committed patchset #1 (id:40001)

[commit-bot: I haz the power, 2014-10-10 02:05:02]

Patchset 1 (id:??) landed as
https://crrev.com/b1d8faa07507442e41c8a8a39217bcd2311b07fa
Cr-Commit-Position: refs/heads/master@{#299034}