ui_devtools: Fix crash when closing while inspecting UI

A DCHECK in ~EventHandler() of OverlayAgentAura checks if the event
handler is removed from its target. Call RemovePreTargetHandler()
in ~OverlayAgentAura() so that this condition is met when closing
the browser window.

Bug: 1163950
Change-Id: Ia8096f057af3bbfc54bc45ccde538dfdb09bf895
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633691
Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844296}

components/ui_devtools/views/overlay_agent_aura.cc

@@ -22,6 +22,7 @@ OverlayAgentAura::OverlayAgentAura(DOMAgent* dom_agent)
 }
 
 OverlayAgentAura::~OverlayAgentAura() {
+  RemovePreTargetHandler();
   overlay_agent_aura_ = nullptr;
 }