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

mojo/edk/system/multiprocess_message_pipe_unittest.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>
 
 #include <string>
@@ skipping 857 matching lines @@
 
     WriteMessage(h, "exit");
   END_CHILD()
 
   CloseHandle(echo_factory_proxy);
   CloseHandle(echo_proxy_a);
   CloseHandle(echo_proxy_b);
   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)
     RUN_CHILD_ON_PIPE(ChannelEchoClient, b)
       VerifyEcho(a, "hello child 0");
       VerifyEcho(b, "hello child 1");
 
       WriteMessage(a, "exit");
       WriteMessage(b, "exit");
     END_CHILD()
   END_CHILD()
 }
-#endif
 
 // Reads and turns a pipe handle some number of times to create lots of
 // transient proxies.
 DEFINE_TEST_CLIENT_TEST_WITH_PIPE(PingPongPipeClient,
                                   MultiprocessMessagePipeTest, h) {
   const size_t kNumBounces = 50;
   MojoHandle p0, p1;
   ReadMessageWithHandles(h, &p0, 1);
   ReadMessageWithHandles(h, &p1, 1);
   for (size_t i = 0; i < kNumBounces; ++i) {
@@ skipping 522 matching lines @@
 INSTANTIATE_TEST_CASE_P(
     ,
     MultiprocessMessagePipeTestWithPeerSupport,
     testing::Values(test::MojoTestBase::LaunchType::CHILD,
                     test::MojoTestBase::LaunchType::PEER,
                     test::MojoTestBase::LaunchType::NAMED_CHILD,
                     test::MojoTestBase::LaunchType::NAMED_PEER));
 }  // namespace
 }  // namespace edk
 }  // namespace mojo