Docserver: Rewrite AvailabilityFinder._GetApiSchemaFilename

This attempts to read //extensions and //chrome API
directories looking for API definitions. FileNotFoundErrors
are ignored in the process.

R=kalman@chromium.org
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/194923010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256537 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256537