Revert of mac: Remove POSIX shared memory.

base/memory/shared_memory.h

Index: base/memory/shared_memory.h
diff --git a/base/memory/shared_memory.h b/base/memory/shared_memory.h
index b7e5a050603d52fce8e41b513c13454b2e5c6be6..13238aabacb1fb4f6924f4bf9b596e9517952861 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -32,7 +32,10 @@
 struct BASE_EXPORT SharedMemoryCreateOptions {
   SharedMemoryCreateOptions();
 
-#if !(defined(OS_MACOSX) && !defined(OS_IOS))
+#if defined(OS_MACOSX) && !defined(OS_IOS)
+  // The type of OS primitive that should back the SharedMemory object.
+  SharedMemoryHandle::Type type;
+#else
   // DEPRECATED (crbug.com/345734):
   // If NULL, the object is anonymous.  This pointer is owned by the caller
   // and must live through the call to Create().
@@ -44,7 +47,7 @@
   // shared memory must not exist.  This flag is meaningless unless
   // name_deprecated is non-NULL.
   bool open_existing_deprecated;
-#endif  // !(defined(OS_MACOSX) && !defined(OS_IOS))
+#endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
   // Size of the shared memory object to be created.
   // When opening an existing object, this has no effect.
@@ -99,7 +102,7 @@
   // The caller is responsible for destroying the duplicated OS primitive.
   static SharedMemoryHandle DuplicateHandle(const SharedMemoryHandle& handle);
 
-#if defined(OS_POSIX) && !(defined(OS_MACOSX) && !defined(OS_IOS))
+#if defined(OS_POSIX)
   // This method requires that the SharedMemoryHandle is backed by a POSIX fd.
   static int GetFdFromSharedMemoryHandle(const SharedMemoryHandle& handle);
 #endif
@@ -119,6 +122,16 @@
   // Creates and maps an anonymous shared memory segment of size size.
   // Returns true on success and false on failure.
   bool CreateAndMapAnonymous(size_t size);
+
+#if defined(OS_MACOSX) && !defined(OS_IOS)
+  // These two methods are analogs of CreateAndMapAnonymous and CreateAnonymous
+  // that force the underlying OS primitive to be a POSIX fd. Do not add new
+  // uses of these methods unless absolutely necessary, since constructing a
+  // fd-backed SharedMemory object frequently takes 100ms+.
+  // http://crbug.com/466437.
+  bool CreateAndMapAnonymousPosix(size_t size);
+  bool CreateAnonymousPosix(size_t size);
+#endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
   // Creates an anonymous shared memory segment of size size.
   // Returns true on success and false on failure.
@@ -244,11 +257,12 @@
   }
 
  private:
-#if defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_ANDROID) && \
-    !(defined(OS_MACOSX) && !defined(OS_IOS))
+#if defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_ANDROID)
   bool PrepareMapFile(ScopedFILE fp, ScopedFD readonly);
+#if !(defined(OS_MACOSX) && !defined(OS_IOS))
   bool FilePathForMemoryName(const std::string& mem_name, FilePath* path);
 #endif
+#endif  // defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_ANDROID)
   enum ShareMode {
     SHARE_READONLY,
     SHARE_CURRENT_MODE,
@@ -268,6 +282,10 @@
   // The OS primitive that backs the shared memory region.
   SharedMemoryHandle shm_;
 
+  // The mechanism by which the memory is mapped. Only valid if |memory_| is not
+  // |nullptr|.
+  SharedMemoryHandle::Type mapped_memory_mechanism_;
+
   int readonly_mapped_file_;
 #elif defined(OS_POSIX)
   int                mapped_file_;