gfx: Use unsigned strides for gpu memory buffers.

services/ui/public/interfaces/gpu_memory_buffer.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module ui.mojom;
 
 import "ui/gfx/mojo/buffer_types.mojom";
 
 struct NativePixmapHandle {
   // A file descriptor for the underlying memory object (usually dmabuf).
   handle fd;
 
   // The stride used when accessing the buffer via a memory mapping.
-  int32 stride;
+  uint32 stride;
 };
 
 
 struct GpuMemoryBufferHandle {
   gfx.mojom.GpuMemoryBufferType type;
   gfx.mojom.GpuMemoryBufferId id;
   handle buffer_handle;
   uint32 offset;
-  int32 stride;
+  uint32 stride;
   NativePixmapHandle? native_pixmap_handle;
   // TODO(fsamuel): Add support for Machports.
 };