Make sure all consumers of the tooltip mixin clean up when unmounted.

tp-tooltip-mixin.js

@@ -8,6 +8,8 @@ import './tp-tooltip.js';
 
 /**
  * Elements implementing this mixin are able to show tooltip on mouse over.
+ * The tooltip is automatically hidden and removed when the element is
+ * disconnected from the DOM.
  *
  * ## Example
  * ```html
@@ -52,6 +54,11 @@ export const Tooltip = function(superClass) {
       return this._tooltip;
     }
 
+    disconnectedCallback() {
+      super.disconnectedCallback();
+      this._hideTooltip();
+    }
+
     _tooltipChanged(newTooltip, oldTooltip) {
       if (oldTooltip) {
         this.removeEventListener('mouseenter', this._showTooltip);