It was passing before r282383, after which it started failing. And after r282490, it's always timing out. The frame rate setting is correctly halved to 30 (calculated by AnimationFrameRate.cpp), but somehow the second requestAnimationFrame callback is called only after the test times out.
Diff of the failure before r282490:
@@ -1,9 +1,10 @@
+ALERT: actual framesPerSecond 61
Test that requestAnimationFrame gets throttled in low power mode.
On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
-PASS framesPerSecond < 36 is true
+FAIL framesPerSecond < 36 should be true. Was false.
PASS successfullyParsed is true