[Pinpoint] Add sign in requirement for try job

Change-Id: I16aaa1acdbb1cade2bb2f1d579dee77d4c9b7e75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2118393Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753112}

docs/speed/perf_trybots.md

@@ -21,7 +21,11 @@ patch applied.
 
 ## Starting a perf try job
 
-Visit [Pinpoint](https://pinpoint-dot-chromeperf.appspot.com) and click the perf try button in the bottom right corner of the screen.
+* Visit [Pinpoint](https://pinpoint-dot-chromeperf.appspot.com).
+* Check the upper-right corner of the page. If you see a "Sign in" link,
+  click it and sign in with an account that has trybot access.
+  (If the link shows "Sign out", then you are already signed in.)
+* Click the perf try button in the bottom right corner of the screen.
 
 ![Pinpoint Perf Try Button](images/pinpoint-perf-try-button.png)