Set surfaceConfiguration alphaMode default value to auto

As raised in [1], the default value for surfaceConfiguration alphaMode
should be "auto", not "opaque".

[1]: emscripten-core/emscripten#21939 (comment)

Bug: dawn:2320
Change-Id: I4201f6ab623ee62a2d206121d0fe2ae1eece67cc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/190080
Reviewed-by: Corentin Wallez <[email protected]>
Commit-Queue: Fr <[email protected]>

src/dawn/dawn.json

@@ -459,7 +459,7 @@
             {"name": "usage", "type": "texture usage", "default": "render attachment"},
             {"name": "view format count", "type": "size_t", "default": 0},
             {"name": "view formats", "type": "texture format", "annotation": "const*", "length": "view format count"},
-            {"name": "alpha mode", "type": "composite alpha mode", "default": "opaque"},
+            {"name": "alpha mode", "type": "composite alpha mode", "default": "auto"},
             {"name": "width", "type": "uint32_t"},
             {"name": "height", "type": "uint32_t"},
             {"name": "present mode", "type": "present mode", "default": "fifo"}