Implement the webgpu.h swapchains on Metal

The webgpu.h surface-based swapchains are implement on Metal which
required adding the present mode to NewSwapChainBase.

Additional automated tests are added which require getting the Instance
so a new getter is added to DawnTest. Additional some the state tracking
of swapchains is performed in the backend, so the
SwapChainValidationTests are turned into regular DawnTests so they can
check backends do the correct state tracking. To not lose coverage of
the Null backend, a NullBackend() DawnTestParam factory is added.

Finally swapchains cannot be entirely tested in an automated fashion, so
a new example is added called "ManualSwapChainTests" that allows
manually checking a number of properties. Documentation of the controls
and a manual test plan is in a comment at the top of the example's
source.

Bug: dawn:269

Change-Id: If62fffc29a6cefdbec62747d01c523e2a5475715
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/17181
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

examples/CHelloTriangle.cpp

@@ -48,7 +48,7 @@ void init() {
 
     const char* fs =
         "#version 450\n"
-        "layout(location = 0) out vec4 fragColor;"
+        "layout(location = 0) out vec4 fragColor;\n"
         "void main() {\n"
         "   fragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
         "}\n";