Docserver: Add clean URL support to the GCS content providers.

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

Index: chrome/common/extensions/docs/templates/json/content_providers.json
diff --git a/chrome/common/extensions/docs/templates/json/content_providers.json b/chrome/common/extensions/docs/templates/json/content_providers.json
index 6d147162439ca00afc962221b88996b103cd87b7..a1448d00826ba8c9231a37c8757ab56b7ad0b60c 100644
--- a/chrome/common/extensions/docs/templates/json/content_providers.json
+++ b/chrome/common/extensions/docs/templates/json/content_providers.json
@@ -79,6 +79,7 @@
     "supportsTemplates": true
   },
   "devtools-docs": {
+    "defaultExtensions": [".html", ".md"],
     "gcs": {
       "bucket": "gs://chromedocs-devtools"
     },
@@ -86,6 +87,7 @@
     "supportsTemplates": true
   },
   "multidevice-docs": {
+    "defaultExtensions": [".html", ".md"],
     "gcs": {
       "bucket": "gs://chromedocs-multidevice"
     },
@@ -93,6 +95,7 @@
     "supportsTemplates": true
   },
   "webstore-docs": {
+    "defaultExtensions": [".html", ".md"],
     "gcs": {
       "bucket": "gs://chromedocs-webstore"
     },