Enable mojo_unittests SharedBufferTest.PassHandleBetweenCousins on Mac.

It passes after the introduction of ChannelMac.

Bug: 973882
Change-Id: I36670b59d94130d916e91471bf48df1f5c7a29b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2329746
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#793278}

mojo/core/shared_buffer_unittest.cc

@@ -194,10 +194,8 @@ DEFINE_TEST_CLIENT_TEST_WITH_PIPE(ReceiveAndEditBufferParent,
   });
 }
 
-#if defined(OS_ANDROID) || defined(OS_MAC)
+#if defined(OS_ANDROID)
 // Android multi-process tests are not executing the new process. This is flaky.
-// Passing shared memory handles between cousins is not currently supported on
-// OSX.
 #define MAYBE_PassHandleBetweenCousins DISABLED_PassHandleBetweenCousins
 #else
 #define MAYBE_PassHandleBetweenCousins PassHandleBetweenCousins