All the buffer backend files had basically the same implemenations
of MapRequestTracker and the tracker was owned by device backends.
This refactor puts MapRequestTracker into its own file
and has the tracker be owned by DeviceBase and BufferBase.
Bug: dawn:400
Change-Id: Id28422b575e9c04d4435d5f119e0ffe08c2d1ce8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/21760
Commit-Queue: Natasha Lee <natlee@microsoft.com>
Reviewed-by: Austin Eng <enga@chromium.org>
