Implement a new child test helper for Android.

mojo/edk/system/message_pipe_unittest.cc

Index: mojo/edk/system/message_pipe_unittest.cc
diff --git a/mojo/edk/system/message_pipe_unittest.cc b/mojo/edk/system/message_pipe_unittest.cc
index 3c53c588be547e525596b438550864a5b43c3312..fcfaeca17904300dca21f92fae0d9f510da7ec58 100644
--- a/mojo/edk/system/message_pipe_unittest.cc
+++ b/mojo/edk/system/message_pipe_unittest.cc
@@ -534,13 +534,7 @@ TEST_F(MessagePipeTest, DISABLED_DataPipeProducerHandlePingPong) {
     MojoClose(p[i]);
 }
 
-#if defined(OS_ANDROID)
-// Android multi-process tests are not executing the new process. This is flaky.
-#define MAYBE_SharedBufferHandlePingPong DISABLED_SharedBufferHandlePingPong
-#else
-#define MAYBE_SharedBufferHandlePingPong SharedBufferHandlePingPong
-#endif
-TEST_F(MessagePipeTest, MAYBE_SharedBufferHandlePingPong) {
+TEST_F(MessagePipeTest, SharedBufferHandlePingPong) {
   MojoHandle buffers[kPingPongHandlesPerIteration];
   for (size_t i = 0; i <kPingPongHandlesPerIteration; ++i)
     EXPECT_EQ(MOJO_RESULT_OK, MojoCreateSharedBuffer(nullptr, 1, &buffers[i]));