On mac SendKeyPressToPlatformApp and ExtensionTestMessageListener don't interact well with each other. There seems to be a delay of 1 key event before the listener can see messages.

Try the sample test as seen here: https://codereview.chromium.org/2657673003/

Run with 

$ ./out/interactive_ui_tests --gtest_filter='ExampleTest.Basic'

on a mac to reproduce.