Add a GUID to base::SharedMemoryHandle.

ui/gfx/mojo/buffer_types_struct_traits.cc

Index: ui/gfx/mojo/buffer_types_struct_traits.cc
diff --git a/ui/gfx/mojo/buffer_types_struct_traits.cc b/ui/gfx/mojo/buffer_types_struct_traits.cc
index 5e3c0fcd6919c0493664b0283ec4bd3f206e5b25..ea726580f9d577167596d06a384f7a388f132654 100644
--- a/ui/gfx/mojo/buffer_types_struct_traits.cc
+++ b/ui/gfx/mojo/buffer_types_struct_traits.cc
@@ -128,11 +128,13 @@ bool StructTraits<gfx::mojom::GpuMemoryBufferHandleDataView,
           mojo::UnwrapPlatformFile(std::move(handle), &platform_file);
       if (unwrap_result != MOJO_RESULT_OK)
         return false;
+      // TODO(rockot): Pass GUIDs through Mojo. https://crbug.com/713763.
+      base::UnguessableToken guid = base::UnguessableToken::Create();

[Ken Rockot(use gerrit already), 2017/05/04 17:54:50]

I don't think this is correct. Note that this deserialization path corresponds
to the serialization path at line 78. That is, these traits don't even serialize
shared memory on Linux using Mojo's shared buffer serialization path, but they
explicitly decompose the shared memory handle into a file descriptor and
serialize that. Mojo internals would have no way of knowing that there's
supposed to be a shm guid associated with the FD in this case, so if you want to
support this now you'd have to add an optional UnguessableToken field to
GpuMemoryBufferHandle which serves as a companion to shared_memory_handle when
used on Linux.

HOWEVER, it seems to me that what you should do instead is change the mojom to
use handle<shared_buffer> for the shared_memory_handle field (so that the
bindings only accept an explicit shared buffer handle rather than a generic Mojo
handle), and then change the Linux de/serialization path to use
WrapSharedMemoryHandle/UnwrapSharedMemoryHandle like Mac does.

I don't know why the code wasn't written that way in the first place, but maybe
the author can explain.

[erikchen, 2017/05/04 19:21:13]

I've left a TODO for myself to update the serialization path, but left the TODO
for you as well.

 #if defined(OS_WIN)
-      out->handle = base::SharedMemoryHandle(platform_file);
+      out->handle = base::SharedMemoryHandle(platform_file, guid);
 #else
-      out->handle =
-          base::SharedMemoryHandle(base::FileDescriptor(platform_file, true));
+      out->handle = base::SharedMemoryHandle(
+          base::FileDescriptor(platform_file, true), guid);
 #endif
 #endif  // defined(OS_MACOSX)
     }