Revert of Disable two tests in mojo_system_unittests that are suspected to be timing out

mojo/edk/system/multiprocess_message_pipe_unittest.cc

Index: mojo/edk/system/multiprocess_message_pipe_unittest.cc
diff --git a/mojo/edk/system/multiprocess_message_pipe_unittest.cc b/mojo/edk/system/multiprocess_message_pipe_unittest.cc
index 897c6d9f59166c709d8adf238bf16b6c943c3fa1..011d824909553c14d4843976959282f5aef5c99b 100644
--- a/mojo/edk/system/multiprocess_message_pipe_unittest.cc
+++ b/mojo/edk/system/multiprocess_message_pipe_unittest.cc
@@ -875,9 +875,6 @@
   CloseHandle(echo_proxy_c);
 }
 
-#if !defined(OS_ANDROID)
-// This test is suspected to be flaky on android https://crbug.com/663998
-// and thus disabled on android.
 TEST_P(MultiprocessMessagePipeTestWithPeerSupport,
        ChannelPipesWithMultipleChildren) {
   RUN_CHILD_ON_PIPE(ChannelEchoClient, a)
@@ -890,7 +887,6 @@
     END_CHILD()
   END_CHILD()
 }
-#endif
 
 // Reads and turns a pipe handle some number of times to create lots of
 // transient proxies.