Make SharedMemoryHandle a class on Mac.

base/memory/BUILD.gn

Index: base/memory/BUILD.gn
diff --git a/base/memory/BUILD.gn b/base/memory/BUILD.gn
index c3442794d8c778dde20ff0636754d2a8660df0a4..79d99ebba70efa27e8cb60aa989e5a39e8e9dfd1 100644
--- a/base/memory/BUILD.gn
+++ b/base/memory/BUILD.gn
@@ -35,6 +35,9 @@ source_set("memory") {
     "scoped_vector.h",
     "shared_memory.h",
     "shared_memory_android.cc",
+    "shared_memory_handle.h",
+    "shared_memory_handle_mac.cc",
+    "shared_memory_mac.cc",
     "shared_memory_nacl.cc",
     "shared_memory_posix.cc",
     "shared_memory_win.cc",
@@ -58,6 +61,10 @@ source_set("memory") {
     sources -= [ "shared_memory_nacl.cc" ]
   }
 
+  if (is_mac) {
+    sources -= [ "shared_memory_posix.cc" ]
+  }
+
   if (is_android) {
     deps = [
       "//third_party/ashmem",