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This patch adds an enum DawnReturnDataType before the return data from
the server side of WebGPU command buffer to the client side. This enum
will be used as a flag to specify different type of return data received
in the client side of WebGPU command buffer. Currently we only support
kDawnCommands, and we will support more kinds of the return values in
the following patches.

BUG=chromium:996713

Change-Id: I99f65a0b91808cea52728ea10b408c4ee9649d85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877907
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710662}