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Define BrowsingDataRemoverDelegate, a class to which BrowsingDataRemover
will delegate all embedder-specific deletions. This class is in 1:1
relationship to BrowsingDataRemover and is owned by it.

Create a Chrome-specific subclass ChromeBrowsingDataRemoverDelegate by
extracting all Chrome datatypes from BrowsingDataRemover::RemoveImpl().

This is the first step of the implementation plan. See the design doc for more
background and explanation of the next steps:
https://docs.google.com/document/d/1I6m4QwbTNhG6wdtazamhTnArJN-UMUGqpvwH6InBEaM/edit#heading=h.m5pzu7ah79n9

This change is covered by BrowsingDataRemoverTest and
BrowsingDataRemoverBrowserTest.

NOTES FOR REVIEWERS:
1. This CL tries to keep the diffs at minimum for easier review, even if the
method ordering could be improved. That will be improved in a follow-up CL.
2. However, the diffs widely differ in BrowsingDataRemover::RemoveImpl and
ChromeBrowsingDataRemoverDelegate::RemoveEmbedderData. Fortunately, one can
convince themself that they are still equivalent by trusting the test coverage
(58 unittests and 6 browsertests).
3. One can convince themself that no datatype was left out by checking the
list of asynchronous tasks, such as "waiting_for_clear_cache_",
"waiting_for_clear_cookies_" etc. present in both .h files and in the AllDone()
method of both classes.
4. The distribution of datatypes might not be final. For example, downloads are
known to content but have a chrome/ dependency. Similarly, the DNS cache lives
in net/ but is retrieved via a class in chrome/. Future CLs might correct their
placement based on how strong the chrome/ dependencies are.

Patchset 3: Simplify the "bool waiting_for_clear_<type>" paradigm by encapsulating it to a SubTask class.

BUG=668114

Review-Url: https://codereview.chromium.org/2554413002
Cr-Commit-Position: refs/heads/master@{#438175}