Disallow nesting on some BrowserThreads.

content/browser/browser_thread_impl.cc

Index: content/browser/browser_thread_impl.cc
diff --git a/content/browser/browser_thread_impl.cc b/content/browser/browser_thread_impl.cc
index a3a33bdea5ecf490df43495f4cba190c0695ff41..5657882dfa97ab16d0fc49f990194889abd97100 100644
--- a/content/browser/browser_thread_impl.cc
+++ b/content/browser/browser_thread_impl.cc
@@ -185,6 +185,14 @@ void BrowserThreadImpl::FlushThreadPoolHelperForTesting() {
 void BrowserThreadImpl::Init() {
   BrowserThreadGlobals& globals = g_globals.Get();
 
+  if (BrowserThread::CurrentlyOn(BrowserThread::DB) ||
+      BrowserThread::CurrentlyOn(BrowserThread::FILE) ||
+      BrowserThread::CurrentlyOn(BrowserThread::FILE_USER_BLOCKING) ||
+      BrowserThread::CurrentlyOn(BrowserThread::PROCESS_LAUNCHER) ||
+      BrowserThread::CurrentlyOn(BrowserThread::CACHE)) {
+    base::MessageLoop::current()->DisallowNesting();
+  }
+
   using base::subtle::AtomicWord;
   AtomicWord* storage =
       reinterpret_cast<AtomicWord*>(&globals.thread_delegates[identifier_]);