This CL add supports for MemoryService choosing implementation based on
ExternalImageType in runtime instead of choosing in compile time.
This CL also removes MemoryServiceNull.cpp.
Bug: dawn:1593
Change-Id: Iddec1f5cf03a7d962ef12b8fd6293ffbb7dc064d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/125900
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shaobo Yan <shaobo.yan@intel.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
