Set marker for client time tracking

Summary:
We didn't start the perf marker with an actual marker, so I don't think we logged the event anywhere.

Searching for `onMessageCallback` doesn't bring anything up in the dev tools.

Reviewed By: danielbuechele

Differential Revision: D15939232

fbshipit-source-id: 95ede47f8a762e33d5532230ba83c2939539ba77

src/Client.js

@@ -125,9 +125,12 @@ export default class Client extends EventEmitter {
         rIC(
           () => {
             const mark = 'onMessageCallback';
-            performance.mark();
-            client.onMessage(payload.data);
-            this.logger.trackTimeSince(mark);
+            performance.mark(mark);
+            try {
+              client.onMessage(payload.data);
+            } finally {
+              this.logger.trackTimeSince(mark);
+            }
           },
           {
             timeout: 500,