Currently http cache uses an old mechanism to detect when it should force enable simple disk_cache instead of the older blockfile cache.  Its doing this by string comparisons looking for "ExperimentYes" groups in a "SimpleCacheTrial" experiment.

The metrics owners have asked that we upgrade this mechanism to use a feature if we expect to continue to use it.

In the most recent finch experiment we have added a "forward compatible" feature called "HttpCacheForceBackend".  It has a "BackendType" param that looks for "simple" in order to force simple disk_cache.

This bug is to make the chromium code actually use this feature.