Update Android Performance for opening trace.html

Summary:
Right now systrace generates an html file that is using the Object.observe function which is now deprecated in most up to date browsers.

Since we don't want to depend on the systrace being up to date, we should advise opening the trace through the Tracing tool, since that is the only way to get it working until systrace removes its dependency on Object.observe.
Closes https://github.com/facebook/react-native/pull/7880

Differential Revision: D3516305

Pulled By: astreet

fbshipit-source-id: ab9ae46ba74b7a34b6cfb8c37ca1ec2e6b41e353

docs/AndroidUIPerformance.md

@@ -45,6 +45,16 @@ After opening the trace in your browser (preferably Chrome), you should see some
 
 ![Example](img/SystraceExample.png)
 
+If your trace .html file isn't opening correctly, check your browser console for the following:
+
+![ObjectObserveError](img/ObjectObserveError.png)
+
+Since Object.observe was deprecated in recent browsers, you may have to open the file from the Google Chrome Tracing tool. You can do so by:
+
+- Opening tab in chrome chrome://tracing
+- Selecting load
+- Selecting the html file generated from the previous command.
+
 **HINT**: Use the WASD keys to strafe and zoom
 
 ### Enable VSync highlighting