Experiment with redirecting all BrowserThreads (but UI/IO) to TaskScheduler

content/public/test/test_browser_thread.cc

Index: content/public/test/test_browser_thread.cc
diff --git a/content/public/test/test_browser_thread.cc b/content/public/test/test_browser_thread.cc
index ae83f78f78aad1455f8a9f98aa6c4f003ef4499b..b9a011b3f2f7ea781a1cd932a6657bee37fb3cd5 100644
--- a/content/public/test/test_browser_thread.cc
+++ b/content/public/test/test_browser_thread.cc
@@ -40,15 +40,31 @@ class TestBrowserThreadImpl : public BrowserThreadImpl {
 };
 
 TestBrowserThread::TestBrowserThread(BrowserThread::ID identifier)
-    : impl_(new TestBrowserThreadImpl(identifier)) {
-}
+    : impl_(new TestBrowserThreadImpl(identifier)), identifier_(identifier) {}
 
 TestBrowserThread::TestBrowserThread(BrowserThread::ID identifier,
                                      base::MessageLoop* message_loop)
-    : impl_(new TestBrowserThreadImpl(identifier, message_loop)) {}
+    : impl_(new TestBrowserThreadImpl(identifier, message_loop)),
+      identifier_(identifier) {}
 
 TestBrowserThread::~TestBrowserThread() {
-  Stop();
+  // The upcoming BrowserThreadImpl::ResetGlobalsForTesting() call requires that
+  // |impl_| have triggered the shutdown phase for its BrowserThread::ID. This
+  // either happens when the thread is stopped (if real) or destroyed (when fake
+  // -- i.e. using an externally provided MessageLoop).
+  impl_.reset();
+
+  // Resets BrowserThreadImpl globals binding |identifier_| to |impl_|. This is
+  // fine since the underlying MessageLoop has already been flushed and deleted
+  // in Stop(). In the case of an externally provided MessageLoop however, this
+  // means that TaskRunners obtained through
+  // |BrowserThreadImpl::GetTaskRunnerForThread(identifier_)| will no longer
+  // recognize their BrowserThreadImpl for RunsTasksOnCurrentThread(). This
+  // happens most often when such verifications are made from
+  // MessageLoop::DestructionObservers. Callers that care to work around that
+  // should instead use this shutdown sequence: TestBrowserThread::Stop() ->
+  // ~MessageLoop() -> ~TestBrowserThread().
+  BrowserThreadImpl::ResetGlobalsForTesting(identifier_);
 }
 
 bool TestBrowserThread::Start() {