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The CacheCounter class in chrome/browser/browsing_data could not be
componentized (unlike other counters in components/browsing_data) because
of its usage of StoragePartitionHttpCacheDataRemover that has a dependency
on content/.

This CL implements a separate CacheCounter class for iOS with the same
functionality. The core of the implementation, a series of IO thread
callbacks, is delegated to the private IOThreadCacheCounter class that
mimics StoragePartitionHttpCacheDataRemover.

BUG=620317

Review-Url: https://codereview.chromium.org/2354643002
Cr-Commit-Position: refs/heads/master@{#420599}