Introduce wgpu::Surface and implement it for HWND, X11 and Metal

This is another step to implement webgpu.h swapchains, Surface is
essentially a union type of all the types of windows that can be used to
create swapchains.

Changes to allow implementing wgpu::Surface and test its creation are:

 - Add GLFWUtils.cpp/.h/_metal.mm  that contains helpers used to use
WebGPU with GLFW. This deprecates BackendBinding.h that will be removed
when the NXT swapchain is removed.
 - Add a `dawn_use_x11` GN variable to factor all the places in BUILD.gn
where we checked whether we should use X11.
 - Add a `supports_glfw_for_windowing` GN variable in the main BUILD.gn
file to control which configuration tests and samples using GLFW can be
built.
 - Add a ObjCUtils.h to contain some ObjC functionality that we'd need
in files that otherwise would be C++ (so that they can be compiled on
all platforms).

Bug: dawn:269

Change-Id: I25548142a1d1d1f05b0f4d71aa3bdc4698d19622
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15081
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

examples/SampleUtils.cpp

@@ -18,6 +18,7 @@
 #include "common/Log.h"
 #include "common/Platform.h"
 #include "utils/BackendBinding.h"
+#include "utils/GLFWUtils.h"
 #include "utils/TerribleCommandBuffer.h"
 
 #include <dawn/dawn_proc.h>