[demos] Add WebGPU demo using origin trial
[demos] Add WebGPU demo using origin trial

Before we get too excited, this does not use CanvasKit.

What this does add is an origin-trial token so we can
more easily demo our progress on *.skia.org and test
locally using localhost:8123 (which is the default
if you run make local in //demos.skia.org)

To view this locally, I started chrome (stable 94) with
--enable-features=Vulkan --enable-unsafe-webgpu

Enabling the Vulkan was the advice of
https://web.dev/gpu/#browser-support
The second flag make sure the whole stack also uses Vulkan.

Change-Id: Ia1838352a46387b0b79e353601a32c859f6911c1
Bug: skia:12512
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/457316
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia