Remove duplicate OnOverviewModeWillStart calls in ToggleOverview

Currently, OnOverviewModeWillStart is called both before and after
ending any pending overview exit animations - we should call this only
once, after existing animations have ended.

Bug: None
Change-Id: I87997a79693de59dbafb572d99799a4c3bb42513
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891390Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710976}

ash/wm/overview/overview_controller.cc

@@ -464,8 +464,6 @@ void OverviewController::ToggleOverview(
   } else {
     DCHECK(CanEnterOverview());
     TRACE_EVENT_ASYNC_BEGIN0("ui", "OverviewController::EnterOverview", this);
-    for (auto& observer : observers_)
-      observer.OnOverviewModeWillStart();
 
     // Clear any animations that may be running from last overview end.
     for (const auto& animation : delayed_animations_)