Implement exponential back-off mechanism.

Implement exponential back-off mechanism.

Contributed by yzshen@google.com, original review http://codereview.chromium.org/4194001/

Implement exponential back-off mechanism. Enforce it at the URLRequestHttpJob level for all outgoing HTTP requests.

The reason why to make this change is that we need back-off logic at a lower enough level to manage all outgoing HTTP traffic, so that the browser won't cause any DDoS attack.

This change:
1) patches http://codereview.chromium.org/2487001/show, which is the exponential back-off implementation.
2) resolves conflicts with URLFetcher, by removing its own back-off logic:
-- removes url_fetcher_protect.{h,cc};
-- integrates the sliding window mechanism of URLFetcherProtectEntry into RequestThrottlerEntry.
3) resolves conflicts with CloudPrintURLFetcher.
4) makes unit tests of CloudPrintURLFetcher, URLFetcher and URLRequest work.

BUG=none
TEST=pass all existing tests and also the newly-added request_throttler_unittest.cc

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=67375