Fix CpuTimeTest to tolerate background test related threads

webrtc/base/cpu_time_unittest.cc

 /*
  *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 44 matching lines @@
   EXPECT_GE(counter1, 0);
   EXPECT_GE(counter2, 0);
   int64_t process_duration_nanos =
       GetProcessCpuTimeNanos() - process_start_time_nanos;
   int64_t thread_duration_nanos =
       GetThreadCpuTimeNanos() - thread_start_time_nanos;
   // This thread did almost nothing.
   // Therefore GetThreadCpuTime is not a wall clock.
   EXPECT_LE(thread_duration_nanos,
             kAllowedErrorMillisecs * kNumNanosecsPerMillisec);
-  // Total process time is twice working threads' CPU time.
+  // Total process time is at least twice working threads' CPU time.
   // Therefore process and thread times are correctly related.
-  EXPECT_NEAR(
+  EXPECT_GE(
       process_duration_nanos,
-      kWorkingThreads * kProcessingTimeMillisecs * kNumNanosecsPerMillisec,
-      kWorkingThreads * kAllowedErrorMillisecs * kNumNanosecsPerMillisec);
+      kWorkingThreads * (kProcessingTimeMillisecs - kAllowedErrorMillisecs)
+      * kNumNanosecsPerMillisec);
 }
 
 TEST(CpuTimeTest, Sleeping) {
   int64_t process_start_time_nanos = GetProcessCpuTimeNanos();
   webrtc::SleepMs(kProcessingTimeMillisecs);
   int64_t process_duration_nanos =
       GetProcessCpuTimeNanos() - process_start_time_nanos;
   // Sleeping should not introduce any additional CPU time.
   // Therefore GetProcessCpuTime is not a wall clock.
   EXPECT_LE(process_duration_nanos,
             kWorkingThreads * kAllowedErrorMillisecs * kNumNanosecsPerMillisec);
 }
 
 }  // namespace rtc