Docserver: Move path canonicalization (the process of detecting and redirecting
mistyped or moved files in the docserver to their canonical locations) to each
content provider so that not only apps and extensions get the benefits but
also NaCL etc.

This is also a prerequisite for implementing the clean URLs referenced the bug,
one of the final P0s for the Chrome-wide docserver launch.

BUG=294008
R=yoz@chromium.org

Review URL: https://codereview.chromium.org/136693022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250312 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250312