Preliminary changes to Vulkan memory suballocation

This mostly makes the MemoryAllocator not owned by the
BuddyResourceAllocator so that we don't need an extra class for the
dependency injection in the Vulkan backend. (the container for the
BuddyMemoryAllocator can be it's MemoryAllocator at the same time).

Also renames methods of MemoryAllocator to be more explicit.

Also renames the constructor parameter of BuddyMemoryAllocator to be
(subjectively) closer to what the represent.

BUG=dawn:27

Change-Id: I37355ad5b3cded143956f0adc4742fa1b717e9bc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12661
Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/ResourceMemoryAllocation.h

@@ -71,4 +71,4 @@ namespace dawn_native {
     };
 }  // namespace dawn_native
 
-#endif  // DAWNNATIVE_RESOURCEMEMORYALLOCATION_H_
+#endif  // DAWNNATIVE_RESOURCEMEMORYALLOCATION_H_