Allow reproduction of the issue in 1156170 with pdfium_test.

Pump the message loop more frequently to expose GC concurency
issues. Does not fix the linked bug but rather makes it reproducible.

To avoid a number of new #ifdefs at various layers, encapsulate
the new logic in a std::function<> callback that can be passed
down to called functions.

Allows removal of a longstanding TODO() in foreground_task.in as
this test now completes as designed.

Bug: chromium:1156170
Change-Id: I82d95d71f31f9c427f330dbf08ab62ba94078079
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/76890
Reviewed-by: Hui Yingst <nigi@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/samples/pdfium_test_event_helper.h b/samples/pdfium_test_event_helper.h
index e823369..f03ddb9 100644
--- a/samples/pdfium_test_event_helper.h
+++ b/samples/pdfium_test_event_helper.h
@@ -5,6 +5,7 @@
 #ifndef SAMPLES_PDFIUM_TEST_EVENT_HELPER_H_
 #define SAMPLES_PDFIUM_TEST_EVENT_HELPER_H_
 
+#include <functional>
 #include <string>
 
 #include "public/fpdf_formfill.h"
@@ -12,6 +13,7 @@
 
 void SendPageEvents(FPDF_FORMHANDLE form,
                     FPDF_PAGE page,
-                    const std::string& events);
+                    const std::string& events,
+                    const std::function<void()>& idler);
 
 #endif  // SAMPLES_PDFIUM_TEST_EVENT_HELPER_H_