Disable PerfTest.AppendResult on iOS.

It seems 'testing::internal::CaptureStdout()' causes problems
when running on real iOS devices.

No-Try: true
Bug: webrtc:8592
Change-Id: Ia7ee636034c6bd1a1ad7a4fb6a2d32e236f64205
Reviewed-on: https://webrtc-review.googlesource.com/27140
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20948}

test/testsupport/perf_test_unittest.cc

@@ -17,7 +17,12 @@
 namespace webrtc {
 namespace test {
 
-TEST(PerfTest, AppendResult) {
+#if defined(WEBRTC_IOS)
+#define MAYBE_AppendResult DISABLED_AppendResult
+#else
+#define MAYBE_AppendResult AppendResult
+#endif
+TEST(PerfTest, MAYBE_AppendResult) {
   testing::internal::CaptureStdout();
   std::string expected = "RESULT measurementmodifier: trace= 42 units\n";
   PrintResult("measurement", "modifier", "trace", 42, "units", false);