FileAPI code should not rely on or assume specific MountPointProvider types

FileAPI code should not rely on or assume specific MountPointProvider types

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*

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=195193

[kinuko, 2013-04-18 11:24:14]

Taiju: Can you take a look at fileapi changes?

Steve: This one's basically fyi, but can you also take a look at media-related
changes if they look ok to you? Thanks!

[tzik, 2013-04-18 14:34:37]

lgtm

[vandebo (ex-Chrome), 2013-04-18 21:32:19]

LGTM

[kinuko, 2013-04-19 14:56:46]

Committed patchset #1 manually as r195193 (presubmit successful).