Add releaseMutexKey to ExternalImageAccessDescriptorDXGIKeyedMutex

This allows the client to specify a different release key for the keyed
mutex than the default of acquire key + 1. If the release key is the
sentinel value of UINT64_MAX, it's set to acquire key + 1. Once Chromium
transitions to always specifying the release key, it will be made a
required parameter and the default behavior will be removed.

Bug: chromium:1213977
Change-Id: I327f7157bb7ff23cf216e43043568ce7c6f38a60
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/53880
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>

src/include/dawn_native/D3D12Backend.h

@@ -56,6 +56,8 @@ namespace dawn_native { namespace d3d12 {
         : ExternalImageAccessDescriptor {
       public:
         uint64_t acquireMutexKey;
+        // Release key will be set to acquireMutexKey + 1 if set to sentinel value UINT64_MAX.
+        uint64_t releaseMutexKey = UINT64_MAX;
         bool isSwapChainTexture = false;
     };