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To move some non-regular MountPointProviders out of webkit/fileapi,
we should not:
- Require FileAPI code change for a new MountPointProvider to
  get/set provider-specific operation context value (e.g.  MediaPathFilter)
- Require a new MountPointProvider type to be listed as a 'frined' of
  FileAPI code

To avoid having these dependencies, this change:
- Makes FileSystemOperationContext inherit from SupportsUserData so that
  MountPointProviders can get/set arbitrary user values with string keys.
- Makes some private methods public (with proper comments) and remove
  the friend list of MountPointProviders

This patch is a part of following change plan (internal only doc):
https://docs.google.com/a/google.com/document/d/1XEtX0OO_RIA_c0KuKvd9yqK5l3XtsTulwXs3c-mAQ1o/view

BUG=175936
TEST=content_unittests:*File*
TEST=browser_tests:*MediaGalleries*

Review URL: https://codereview.chromium.org/14341004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@195193 0039d316-1c4b-4281-b951-d872f2087c98