[tools] Discard tooltip on hide

Bug: v8:10644
Change-Id: I98a557782cabadec3a85ca04cd3fc6c391e239d7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2565352Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71490}

tools/system-analyzer/view/tool-tip.mjs

@@ -91,6 +91,7 @@ DOM.defineCustomElement(
       hide() {
         this._isHidden = true;
         this.bodyNode.style.display = 'none';
+        this.targetNode = undefined;
       }
 
       show() {