Add POSIX shared memory support for Mac

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 3d21aa4d350f80608c2feb07147b38de405386cb..4c09c2bca43933f952584f716f76dfc5824c8349 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -511,6 +511,8 @@ component("base") {
     "memory/shared_memory_handle.h",
     "memory/shared_memory_handle_mac.cc",
     "memory/shared_memory_handle_win.cc",
+    "memory/shared_memory_helper.cc",
+    "memory/shared_memory_helper.h",
     "memory/shared_memory_mac.cc",
     "memory/shared_memory_nacl.cc",
     "memory/shared_memory_posix.cc",
@@ -1165,6 +1167,8 @@ component("base") {
       "memory/discardable_memory_allocator.h",
       "memory/discardable_shared_memory.cc",
       "memory/discardable_shared_memory.h",
+      "memory/shared_memory_helper.cc",
+      "memory/shared_memory_helper.h",
       "memory/shared_memory_posix.cc",
       "native_library.cc",
       "native_library_posix.cc",