Bug 1159751: Ensure WARP can never be used for Windows 7. r=milan

gfx/thebes/gfxWindowsPlatform.cpp

@@ -1883,6 +1883,11 @@ gfxWindowsPlatform::InitD3D11Devices()
   }
 
   bool useWARP = false;
+  bool allowWARP = false;
+
+  if (IsWin8OrLater()) {
+    allowWARP = true;
+  }
 
   nsCOMPtr<nsIGfxInfo> gfxInfo = do_GetService("@mozilla.org/gfx/info;1");
   if (gfxInfo) {
@@ -1906,7 +1911,7 @@ gfxWindowsPlatform::InitD3D11Devices()
             }
         }
 
-        useWARP = true;
+        useWARP = allowWARP;
       }
     }
   }
@@ -1942,7 +1947,7 @@ gfxWindowsPlatform::InitD3D11Devices()
       if (!gfxPrefs::LayersD3D11DisableWARP()) {
         return;
       }
-      useWARP = true;
+      useWARP = allowWARP;
     }
   }
 
@@ -1964,7 +1969,7 @@ gfxWindowsPlatform::InitD3D11Devices()
         return;
       }
 
-      useWARP = true;
+      useWARP = allowWARP;
       adapter = nullptr;
     }
 
@@ -1974,7 +1979,7 @@ gfxWindowsPlatform::InitD3D11Devices()
         return;
       }
 
-      useWARP = true;
+      useWARP = allowWARP;
       adapter = nullptr;
     }