Extract some logic from transport_dib into 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 692ccf8a67cf6fbbce1dcc7682a1a347c254aba7..e288058942bd35f6cae7ba798c9b94ec80d43d53 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -130,6 +130,12 @@ class BASE_EXPORT SharedMemory {
       bool clean_up_resources_on_destruction);
 #endif
 
+#if defined(OS_POSIX) && !defined(OS_ANDROID)
+  // Returns the size of the shared memory region referred to by |handle|.
+  // Returns '-1' on a failure to determine the size.
+  static int GetSizeFromSharedMemoryHandle(const SharedMemoryHandle& handle);
+#endif  // defined(OS_POSIX) && !defined(OS_ANDROID)
+
   // Creates a shared memory object as described by the options struct.
   // Returns true on success and false on failure.
   bool Create(const SharedMemoryCreateOptions& options);