Bug 1262427. Don't try D3D11 harder. r=dvander

If we know D3D11 isn't going to work we shouldn't bother
using it for ANGLE.

gfx/gl/GLLibraryEGL.cpp

@@ -179,7 +179,7 @@ GetAndInitDisplayForAccelANGLE(GLLibraryEGL& egl)
             return GetAndInitDisplay(egl, LOCAL_EGL_D3D11_ONLY_DISPLAY_ANGLE);
 
         if (gfxPrefs::WebGLANGLETryD3D11() &&
-            gfxPlatform::CanUseDirect3D11ANGLE())
+            gfxPlatform::GetPlatform()->CanUseDirect3D11ANGLE())
         {
             ret = GetAndInitDisplay(egl, LOCAL_EGL_D3D11_ELSE_D3D9_DISPLAY_ANGLE);
         }

gfx/thebes/gfxPlatform.cpp

@@ -2173,13 +2173,6 @@ gfxPlatform::CanUseHardwareVideoDecoding()
   return sLayersSupportsHardwareVideoDecoding && !sLayersHardwareVideoDecodingFailed;
 }
 
-bool
-gfxPlatform::CanUseDirect3D11ANGLE()
-{
-  MOZ_ASSERT(sLayersAccelerationPrefsInitialized);
-  return gANGLESupportsD3D11;
-}
-
 bool
 gfxPlatform::AccelerateLayersByDefault()
 {

gfx/thebes/gfxPlatform.h

@@ -451,7 +451,7 @@ public:
 
     static bool CanUseDirect3D9();
     virtual bool CanUseHardwareVideoDecoding();
-    static bool CanUseDirect3D11ANGLE();
+    virtual bool CanUseDirect3D11ANGLE() { return false; }
 
     // Returns a prioritized list of all available compositor backends.
     void GetCompositorBackends(bool useAcceleration, nsTArray<mozilla::layers::LayersBackend>& aBackends);

gfx/thebes/gfxWindowsPlatform.cpp

@@ -428,6 +428,12 @@ gfxWindowsPlatform::CanUseHardwareVideoDecoding()
   return !IsWARP() && gfxPlatform::CanUseHardwareVideoDecoding();
 }
 
+bool
+gfxWindowsPlatform::CanUseDirect3D11ANGLE()
+{
+  return gANGLESupportsD3D11 && gfxConfig::IsEnabled(Feature::D3D11_COMPOSITING);
+}
+
 bool
 gfxWindowsPlatform::InitDWriteSupport()
 {

gfx/thebes/gfxWindowsPlatform.h

@@ -97,6 +97,8 @@ struct ClearTypeParameterInfo {
     int32_t     enhancedContrast;
 };
 
+extern bool gANGLESupportsD3D11;
+
 class gfxWindowsPlatform : public gfxPlatform {
 public:
     enum TextRenderingMode {
@@ -260,6 +262,8 @@ public:
     }
     bool SupportsPluginDirectDXGIDrawing();
 
+    virtual bool CanUseDirect3D11ANGLE();
+
 protected:
     bool AccelerateLayersByDefault() override {
       return true;