[mojo-edk] Switch shared buffers to use Mach on OSX.

BUG=582468

Review URL: https://codereview.chromium.org/1804353002

Cr-Commit-Position: refs/heads/master@{#381785}

mojo/edk/embedder/platform_shared_buffer.cc

@@ -175,8 +175,7 @@ bool PlatformSharedBuffer::Init() {
   base::SharedMemoryCreateOptions options;
   options.size = num_bytes_;
 #if defined(OS_MACOSX) && !defined(OS_IOS)
-  // TODO(crbug.com/582468): Support Mach shared memory.
-  options.type = base::SharedMemoryHandle::POSIX;
+  options.type = base::SharedMemoryHandle::MACH;
 #endif
 
   shared_memory_.reset(new base::SharedMemory);

mojo/edk/system/shared_buffer_unittest.cc

@@ -216,8 +216,10 @@ DEFINE_TEST_CLIENT_TEST_WITH_PIPE(ReceiveAndEditBufferParent, SharedBufferTest,
   END_CHILD()
 }
 
-#if defined(OS_ANDROID)
+#if defined(OS_ANDROID) || defined(OS_MACOSX)
 // 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