Enable EGL_ANGLE_keyed_mutex on D3D11

src/libANGLE/renderer/d3d/d3d11/Renderer11.cpp

@@ -1078,10 +1078,7 @@ void Renderer11::generateDisplayExtensions(egl::DisplayExtensions *outExtensions
         outExtensions->surfaceD3DTexture2DShareHandle = true;
     }
 
-#ifdef ANGLE_ENABLE_KEYEDMUTEX
     outExtensions->keyedMutex = true;
-#endif
-
     outExtensions->querySurfacePointer = true;
     outExtensions->windowFixedSize     = true;
     outExtensions->surfaceOrientation  = true;

src/libANGLE/renderer/d3d/d3d11/SwapChain11.cpp

@@ -263,9 +263,11 @@ EGLint SwapChain11::resetOffscreenColorBuffer(int backbufferWidth, int backbuffe
                 }
             }
         }
-        mKeyedMutex = d3d11::DynamicCastComObject<IDXGIKeyedMutex>(mOffscreenTexture);
     }
 
+    // This may return null if the original texture was created without a keyed mutex.
+    mKeyedMutex = d3d11::DynamicCastComObject<IDXGIKeyedMutex>(mOffscreenTexture);
+
     D3D11_RENDER_TARGET_VIEW_DESC offscreenRTVDesc;
     offscreenRTVDesc.Format = backbufferFormatInfo.rtvFormat;
     offscreenRTVDesc.ViewDimension = D3D11_RTV_DIMENSION_TEXTURE2D;