Docserver: Add clean URL support to the GCS 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.
@@ skipping 61 matching lines @@
   },
   "cr-native-client": {
     "chromium": {
       "dir": "native_client_sdk/doc_generated"
     },
     "defaultExtensions": [".html", ".md"],
     "serveFrom": "native-client",
     "supportsTemplates": true
   },
   "devtools-docs": {
+    "defaultExtensions": [".html", ".md"],
     "gcs": {
       "bucket": "gs://chromedocs-devtools"
     },
     "serveFrom": "devtools",
     "supportsTemplates": true
   },
   "multidevice-docs": {
+    "defaultExtensions": [".html", ".md"],
     "gcs": {
       "bucket": "gs://chromedocs-multidevice"
     },
     "serveFrom": "multidevice",
     "supportsTemplates": true
   },
   "webstore-docs": {
+    "defaultExtensions": [".html", ".md"],
     "gcs": {
       "bucket": "gs://chromedocs-webstore"
     },
     "serveFrom": "webstore",
     "supportsTemplates": true
   }
 }