Make OAuth client id accessible to new ui w/o hardcoding

appengine/swarming/elements/res/imp/botlist/bot-list.html

Index: appengine/swarming/elements/res/imp/botlist/bot-list.html
diff --git a/appengine/swarming/elements/res/imp/botlist/bot-list.html b/appengine/swarming/elements/res/imp/botlist/bot-list.html
index 9522227e3afd73e250fbedadaa4bf236473379bf..9dae79c2f5935b6c5f1de3cc84fb5238b8c1a59e 100644
--- a/appengine/swarming/elements/res/imp/botlist/bot-list.html
+++ b/appengine/swarming/elements/res/imp/botlist/bot-list.html
@@ -11,8 +11,10 @@
   bot-list creats a dynamic table for viewing swarming bots.  Columns can be
   dynamically filtered and it supports client-side filtering.
 
+  This is a top-level element.
+
   Properties:
-    None.  This is a top-level element.
+    client_id: String, will be set by server-side template evaluation.
 
   Methods:
     None.
@@ -76,6 +78,7 @@
     </style>
 
     <swarming-app
+      client_id="[[client_id]]"
       auth_headers="{{_auth_headers}}"
       signed_in="{{_signed_in}}"
 
@@ -383,6 +386,10 @@
 
       properties: {
 
+        client_id: {
+          type: String,
+        },
+
         _bots: {
           type: Array,
         },