Add the webgpu.h swapchain creation path

This commit changes wgpu::Device::CreateSwapChain to take an additional
wgpu::Surface argument. Passing nullptr is enough to stay on the
previous swapchain implementation, until the new one is ready.

In order to support both the "old" implementation-based swapchains and
the "new" surface-based swapchains. SwapChainBase is now split into
three abstract classes:

 - SwapChainBase that has a virtual method for each of the
wgpu::SwapChain methods.
 - OldSwapChainBase that corresponds to the implementation-based
swapchains.
 - NewSwapChainBase that will contain the surface-based swapchain
implementation and will eventually just be renamed to SwapChainBase.

The interaction of the surface-based swapchains with the Surface objects
aren't implemented yet, neither are the swapchain methods. Only creation
works.

Validation tests for surface-based swapchain creation are added in the
end2end test target because they need to create OS windows.

Bug: dawn:269

Change-Id: I7e07d6c666479867b9a16d7b1b8c181d5dbd69a0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15281
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
diff --git a/examples/CHelloTriangle.cpp b/examples/CHelloTriangle.cpp
index d457e38..0d36090 100644
--- a/examples/CHelloTriangle.cpp
+++ b/examples/CHelloTriangle.cpp
@@ -33,7 +33,7 @@
         descriptor.nextInChain = nullptr;
         descriptor.label = nullptr;
         descriptor.implementation = GetSwapChainImplementation();
-        swapchain = wgpuDeviceCreateSwapChain(device, &descriptor);
+        swapchain = wgpuDeviceCreateSwapChain(device, nullptr, &descriptor);
     }
     swapChainFormat = static_cast<WGPUTextureFormat>(GetPreferredSwapChainTextureFormat());
     wgpuSwapChainConfigure(swapchain, swapChainFormat, WGPUTextureUsage_OutputAttachment, 640, 480);