Real instance methods for BrowserContext: GetBrowsingDataRemover.

This CL refactors non-overridable (i.e. implemented only within
//content) methods of `BrowserContext`, so that they are implemented as
instance methods (rather than as `static` methods taking a `self`
argument).  This particular CL refactors the following methods:
- GetBrowsingDataRemover

For the rationale/motivation for this change, please see the CL
description of an earlier CL in this series - r877482.

Bug: 1179776
Change-Id: I330e46b997d54ae11de8728ab3548b8edfbc77a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2880841


Commit-Queue: Charlie Reis <creis@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Owners-Override: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882429}