Fix order-dependent tests.

webrtc::test::GetPerfResults() relies on a singleton and this makes
some tests be order dependent (running in a different order makes them
fail).

A good fix is to remove the singleton but this CL at least makes the
fragile test set up the environment correctly.

No-Try: True
Bug: None
Change-Id: I7ad25f685f0bc5d246beeadedfa9f5a39f3547e5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/197425
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Andrey Logvin <landrey@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32823}

test/testsupport/perf_test_unittest.cc

@@ -64,6 +64,7 @@ TEST_F(PerfTest, TestClearPerfResults) {
 #if WEBRTC_ENABLE_PROTOBUF
 
 TEST_F(PerfTest, TestGetPerfResultsHistograms) {
+  ClearPerfResults();
   PrintResult("measurement", "_modifier", "story_1", 42, "ms", false);
   PrintResult("foo", "bar", "story_1", 7, "sigma", true);
   // Note: the error will be ignored, not supported by histograms.