Residency 2: Create a d3d12::Heap During Direct Allocations

When creating a directly allocated resource in D3D12, also create a
dawn_native::d3d12::Heap to represent that allocation alongside the
ResourceHeapAllocation. This matches D3D12's allocation model when using
CreateCommittedResource and makes residency management much easier.

Bug: dawn:193
Change-Id: I2280863dcfca57bad72962a2b097f8f2d4cc7dad
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16381
Commit-Queue: Brandon Jones <brandon1.jones@intel.com>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/ResourceMemoryAllocation.h

@@ -31,6 +31,9 @@ namespace dawn_native {
         // Memory sub-divided using one or more blocks of various sizes.
         kSubAllocated,
 
+        // Memory was allocated outside of Dawn.
+        kExternal,
+
         // Memory not allocated or freed.
         kInvalid
     };