Add notification for aborted popups (fixes #3776)

Pass a new |popup_id| parameter to OnBeforePopup and call a new
OnBeforePopupAborted callback if the popup is aborted before
OnAfterCreated is called for the popup browser. Add new
CefBrowserHost::GetBrowserByIdentifier and GetOpenerIdentifier
methods to assist with retrieval of associated browsers.

In cefclient, clean up state when a popup is aborted and close
any associated popup browsers when the opener browser is closed.
This also works when running with `--use-default-popup`.

libcef/browser/browser_host_create.cc

@@ -151,6 +151,21 @@ CefRefPtr<CefBrowser> CefBrowserHost::CreateBrowserSync(
   return CefBrowserHostBase::Create(create_params);
 }
 
+// static
+CefRefPtr<CefBrowser> CefBrowserHost::GetBrowserByIdentifier(int browser_id) {
+  // Verify that the context is in a valid state.
+  if (!CONTEXT_STATE_VALID()) {
+    DCHECK(false) << "context not valid";
+    return nullptr;
+  }
+
+  if (browser_id <= 0) {
+    return nullptr;
+  }
+
+  return CefBrowserHostBase::GetBrowserForBrowserId(browser_id).get();
+}
+
 // static
 bool CefBrowserCreateParams::IsChromeStyle(const CefWindowInfo* window_info) {
   if (!window_info) {