Add FlushForTesting to base::Thread.

base/threading/thread.cc

Index: base/threading/thread.cc
diff --git a/base/threading/thread.cc b/base/threading/thread.cc
index 6181ba66918304d713d79dcbc3e0970276e9ea07..761bc871885ede525866ca4a6c3e6226cb0e3005 100644
--- a/base/threading/thread.cc
+++ b/base/threading/thread.cc
@@ -144,6 +144,18 @@ bool Thread::WaitUntilThreadStarted() const {
   return true;
 }
 
+void Thread::FlushForTesting() {

[danakj, 2016/09/15 18:09:33]

I'd prefer a free function in base/test/ actually. There's no reason this has to
be on Thread right?

[gab, 2016/09/15 20:18:33]

Maybe discoverability is a reason? i.e. I just realized that such a thing
already more or less exists
(https://cs.chromium.org/chromium/src/base/test/thread_test_helper.h) ...
whereas I've always seen "FlushForTesting" methods on APIs themselves and never
thought of looking in base/test for this.

WDYT?

+  DCHECK(owning_sequence_checker_.CalledOnValidSequence());
+  if (!message_loop_)
+    return;
+
+  WaitableEvent done(WaitableEvent::ResetPolicy::AUTOMATIC,
+                     WaitableEvent::InitialState::NOT_SIGNALED);
+  task_runner()->PostTask(FROM_HERE,
+                          Bind(&WaitableEvent::Signal, Unretained(&done)));
+  done.Wait();
+}
+
 void Thread::Stop() {
   DCHECK(joinable_);