Docserver: Enable GitHub content providers.

chrome/common/extensions/docs/templates/json/content_providers.json

 // === Overview ===
 //
 // This file configures where to find and how to serve content in the docserver.
 // It's the most fundamentally important file in all of the docserver.
 //
 // === Format ===
 //
 // Each entry declares a rule with:
 //  * An arbitrary identifier key e.g. "cr-extensions-examples".
 //  * What URL the rule should be invoked with, given by "serveFrom", e.g.
 //    "extensions/examples".
-//  * An object describing where the content originates; currently the only
-//    supported location is "chromium" with a key "dir" specifying which
-//    chromium directory to look in, e.g. "extensions/samples".
+//  * An object describing where the content originates, either "chromium"
+//    or "github".
+//  * "chromium" must provide a "dir" value specifying which chromium directory
+//     to look in, e.g. "extensions/samples".
+//  * "github" must provide "owner" and "repo" values specifying the owner of
+//    the GitHub repository, and the repository name, e.g. "GoogleChrome" and
+//    "chrome-app-samples" respectively.
 //
-// In this example, when the user navigates to
+// In the chromium example, when the user navigates to
 //
 //    developer.chrome.com/extensions/examples/some/sample/path
 //
 // then
 //  1. The "cr-extensions-examples" rule is invoked (since it's served from
 //     "extensions/examples").
 //  2. The docserver will look up the path "docs/examples/some/sample/path"
 //     in chromium - the URL after the "serveFrom" path (some/sample/path)
 //     prefixed by the chromium directory (docs/examples).
 //  3. Then render and serve it.
@@ skipping 24 matching lines @@
     "serveFrom": "",
     "supportsTemplates": true
   },
   "cr-static": {
     "chromium": {
       "dir": "docs/static"
     },
     "serveFrom": "static"
   }
 }