This CL introduced ServiceImplementation class. Service class deleate
its work to ServiceImplementation class.
Different platforms inherit ServiceImplementation class and implement
it to support Service class. It helps isolate implementaion and
interface. And it's the base for supporting choosing implementation
in runtime.
Bug: dawn:1593
Change-Id: Ib18dfaa41ef4a1ce9554a5241cac1ef0fede8dd8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/125481
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shaobo Yan <shaobo.yan@intel.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
