Add testRunner.runIdleTasks() to force idle tasks to run.

content/test/layouttest_support.cc

Index: content/test/layouttest_support.cc
diff --git a/content/test/layouttest_support.cc b/content/test/layouttest_support.cc
index 5ec52f637b519467b6ddf013663c7ba3cf6cd587..9b9e75107f5ef9feb5410dcf3b6853735f14f4c8 100644
--- a/content/test/layouttest_support.cc
+++ b/content/test/layouttest_support.cc
@@ -11,6 +11,7 @@
 #include "base/lazy_instance.h"
 #include "base/strings/string_util.h"
 #include "build/build_config.h"
+#include "components/scheduler/test/renderer_scheduler_test_support.h"
 #include "components/test_runner/test_common.h"
 #include "components/test_runner/web_frame_test_proxy.h"
 #include "components/test_runner/web_test_proxy.h"
@@ -456,4 +457,10 @@ std::string DumpBackForwardList(std::vector<PageState>& page_state,
   return result;
 }
 
+void SchedulerRunIdleTasks(const base::Closure& callback) {
+    scheduler::RendererScheduler* scheduler =
+        content::RenderThreadImpl::current()->GetRendererScheduler();
+    scheduler::RunIdleTasksForTesting(scheduler, callback);
+}
+
 }  // namespace content