Implement GPUBufferDescriptor.mappedAtCreation.

This CL:

 - Adds mappedAtCreation to dawn.json
 - Changes dawn_native to implement CreateBufferMapped in terms of
   mappedAtCreation.
 - Duplicates all the CreateBufferMappedTests to mappedAtCreation tests
   (both validation and end2end).
 - Implements dawn_wire's mappedAtCreation in terms of
   CreateBufferMapped. The reversal in dawn_wire will be done in a
   follow-up CL.

Bug: dawn:445

Change-Id: I70b9fa729b1402524a6b993c3f288987eb65c6c4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/24083
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>