Files.app: Conver private API scheme from JSON to IDL.

To describe the scheme of extension API, we can use both JSON and IDL. Since IDL is more readable than JSON, we switched the format into IDL.

BUG=247260
TEST=FileManagerBrowserTest

Review URL: https://codereview.chromium.org/112803002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240637 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240637