Document common causes of flaky GTests

This patch lists some common causes of flakes in GTests, and links to
examples that might be helpful. Credit for the examples goes to the
security architecture team plus an example of my own.

Bug: 1127370
Change-Id: Ied695425ebab5935ef54094972fdbc1bfa9f187d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559271Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Nick Burris <nburris@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831889}

docs/testing/gtest_flake_tips.md

@@ -58,12 +58,20 @@ reproduce the flake with more information.
 If the test is flakily timing out, consider any asynchronous code that may cause
 race conditions, where the test subject may early exit and miss a callback, or
 return faster than the test can start waiting for it (i.e. make sure event
-listeners are spawned before invoking the event).
+listeners are spawned before invoking the event). Make sure event listeners are
+for the proper event instead of a proxy (e.g. [Wait for the correct event in
+test](https://chromium.googlesource.com/chromium/src/+/6da09f7510e94d2aebbbed13b038d71c511d6cbc)).
+
+Consider possible bugs in the system or test infrastructure (e.g. [races in
+glibc](https://bugs.chromium.org/p/chromium/issues/detail?id=1010318)).
 
 For browsertest flakes, consider possible inter-process issues, such as the
-renderer taking too long or returning something unexpected.
+renderer taking too long or returning something unexpected (e.g. [flaky
+RenderFrameHostImplBrowserTest](https://bugs.chromium.org/p/chromium/issues/detail?id=1120305)).
 
->TODO: Add more tips for common flake causes
+For browsertest flakes that check EvalJs results, make sure test objects are not
+destroyed before JS may read their values (e.g. [flaky
+PaymentAppBrowserTest](https://chromium.googlesource.com/chromium/src/+/6089f3480c5036c73464661b3b1b6b82807b56a3)).
 
 ## Preventing similar flakes