Add tests and fix (create) mapping zero-sized buffers

When creating a zero-sized buffer mapped, StagingBuffer creation
is skipped. This required adding a new MappedAtCreation state
since mStagingBuffer couldn't be used as a tag value for that.

Made the OpenGL backend always create non-zero-sized buffers.

Finally added tests for MapRead/WriteAsync and CreateBufferMapped
of zero-sized buffers.

Bug: dawn:446
Change-Id: I04f6fe98fd646f1867c21065cd1cd33a1595e19f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/21481
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_wire/server/ServerBuffer.cpp

@@ -202,6 +202,7 @@ namespace dawn_wire { namespace server {
             // to Unmap and attempt to update mapped data of an error buffer.
             return false;
         }
+
         // Deserialize the flush info and flush updated data from the handle into the target
         // of the handle. The target is set via WriteHandle::SetTarget.
         return buffer->writeHandle->DeserializeFlush(writeFlushInfo,