Move WebGPU context creation from GPUDevice to NavigatorGPU

nagivator.gpu should hold the WebGPU context. The context needs
to be initialized before any GPUAdapters or GPUDevices are created.

Bug: 941531
Change-Id: Ie70416a59c09b49fe799365e2c17a7616cbb0609
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1521234
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640969}
10 files changed