Perf script runner Python 3 compatibility.

No code changes in ANGLE, just a perf test runner fix.

BUG=angleproject:1155

Change-Id: If49c58da26f9224e09f292689b2d74f7e129a844
Reviewed-on: https://chromium-review.googlesource.com/633786
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

scripts/perf_test_runner.py

@@ -100,7 +100,7 @@ print('Test name: ' + test_name)
 
 # Infinite loop of running the tests.
 while True:
-    output = subprocess.check_output([perftests_path, '--gtest_filter=' + test_name])
+    output = subprocess.getoutput([perftests_path, '--gtest_filter=' + test_name])
 
     start_index = output.find(metric + "=")
     if start_index == -1: