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This patch is the first one to implement requesting a GPUDevice with a
descriptor that indicates the required extensions on this device. With
this patch a WGPUDeviceDescriptor is sent from Blink to the GPU process
so that we can specify the required extensions when creating the Dawn
Device object.

In the next patch we will add the callback to receive the device id
after the creation of the Dawn Device object in the GPU process.

BUG=chromium:996713

Change-Id: If0ce899b17f36fe3f87af497d2e6af1189cc5a3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1938834
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721882}