WebLayer: clear fullscreen exit closure for renderer-initiated exits

Bug: none
Change-Id: I0dda6eb22f1bbdb6f11b4da0818ca34adb259728
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086688
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746903}

weblayer/browser/fullscreen_callback_proxy.cc

@@ -35,6 +35,9 @@ void FullscreenCallbackProxy::EnterFullscreen(base::OnceClosure exit_closure) {
 
 void FullscreenCallbackProxy::ExitFullscreen() {
   TRACE_EVENT0("weblayer", "Java_FullscreenCallbackProxy_exitFullscreen");
+  // If the web contents initiated the fullscreen exit, the closure will still
+  // be valid, so clean it up now.
+  exit_fullscreen_closure_.Reset();
   Java_FullscreenCallbackProxy_exitFullscreen(AttachCurrentThread(),
                                               java_delegate_);
 }