Implement delete methods for the various CannedBrowsingDataHelpers.

This will allow individual cookies and other site data to be deleted via the
CookieTreeModel GUI. As a result of this change, updates to the various
storage systems will be pushed to the canned helpers after creation of the
CookieTreeModel.

Also removes a pimpl for BrowsingDataIndexedDBHelper since no other canned
helpers followed this pattern.

BUG=168996
TEST=No visible changes yet. Deleters covered by unit tests.
TBR=dbeam@chromium.org

Review URL: https://codereview.chromium.org/124183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243315 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243315