Increase D3D12 Swap Chain Buffers to 3

Adds an additional buffer and frame to the D3D12 swap chain to
unthrottle fps in situations where we can't maintain a constant 60fps.

Bug: dawn:142
Change-Id: I06ef697ca37608dc51816dd99de5534cb8c9b843
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/6840
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/d3d12/NativeSwapChainImplD3D12.cpp

@@ -35,7 +35,7 @@ namespace dawn_native { namespace d3d12 {
             return usage;
         }
 
-        static constexpr unsigned int kFrameCount = 2;
+        static constexpr unsigned int kFrameCount = 3;
     }  // anonymous namespace
 
     NativeSwapChainImpl::NativeSwapChainImpl(Device* device, HWND window)