Auto merge of #6161 - jgraham:webdriver_stability, r=glennw

This fixes various race conditions that affect test execution when using the servodriver product. It doesn't yet do enough to make servodriver a viable alternative to the normal servo test executor.

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components/compositing/compositor.rs

@@ -1281,6 +1281,9 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 // Constellation has replied at some point in the past
                 // that the current output image is stable and ready
                 // for saving.
+                // Reset the flag so that we check again in the future
+                // TODO: only reset this if we load a new document?
+                self.ready_to_save_state = ReadyState::Unknown;
                 true
             }
         }