Clear previews_freshness_ on new navigations

This ensures that an old previews_freshness_ won't be shown on the
wrong page.

Bug: 898570
Change-Id: I19cb1ab9916eae9b80fb4ca8d260cd18ebd66a97
Reviewed-on: https://chromium-review.googlesource.com/c/1298245Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602528}

chrome/browser/previews/previews_ui_tab_helper.cc

@@ -240,6 +240,7 @@ void PreviewsUITabHelper::DidFinishNavigation(
       !navigation_handle->HasCommitted() || navigation_handle->IsSameDocument())
     return;
 
+  previews_freshness_ = base::Time();
   previews_user_data_.reset();
 #if defined(OS_ANDROID)
   should_display_android_omnibox_badge_ = false;