Made MessagePump a non-thread safe class.

media/base/pipeline_unittest.cc

Index: media/base/pipeline_unittest.cc
diff --git a/media/base/pipeline_unittest.cc b/media/base/pipeline_unittest.cc
index 804fc9916f8d66673bfe5883efee64b53e18d2fa..90c616f92db80c8207b8924f4357c28f59ecd365 100644
--- a/media/base/pipeline_unittest.cc
+++ b/media/base/pipeline_unittest.cc
@@ -689,14 +689,14 @@ static void TestNoCallsAfterError(
   CHECK(message_loop);
 
   // When we get to this stage, the message loop should be empty.
-  message_loop->AssertIdle();
+  EXPECT_TRUE(message_loop->IsIdleForTesting());
 
   // Make calls on pipeline after error has occurred.
   pipeline->SetPlaybackRate(0.5f);
   pipeline->SetVolume(0.5f);
 
   // No additional tasks should be queued as a result of these calls.
-  message_loop->AssertIdle();
+  EXPECT_TRUE(message_loop->IsIdleForTesting());
 }
 
 TEST_F(PipelineTest, NoMessageDuringTearDownFromError) {