Bug 664764. Part 1: Change assertion to warning since it can happen due to resource exhaustion. r=mattwoodrow

gfx/layers/basic/BasicLayers.cpp

@@ -766,9 +766,10 @@ BasicThebesLayer::PaintThebes(gfxContext* aContext,
       RenderTraceInvalidateEnd(this, "FFFF00");
     } else {
       // It's possible that state.mRegionToInvalidate is nonempty here,
-      // if we are shrinking the valid region to nothing.
-      NS_ASSERTION(state.mRegionToDraw.IsEmpty(),
-                   "If we need to draw, we should have a context");
+      // if we are shrinking the valid region to nothing. So use mRegionToDraw
+      // instead.
+      NS_WARN_IF_FALSE(state.mRegionToDraw.IsEmpty(),
+                       "No context when we have something to draw; resource exhaustion?");
     }
   }