Docserver: fix small portability bug in path_canonicalizer

Depending on platform, an invalid path may be canonicalized differently
depending on the order paths are read from the file system. This causes
unittests to sometimes fail.

Now paths are ordered first by length then lexicographically, so
apps/storage.html is always suggested over extensions/storage.html and
app/manifest/storage.html.

NOTRY=true

Review URL: https://codereview.chromium.org/159973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250485 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250485