Consider PlatformThread::Join() to be blocking IO.

chrome/browser/browser_process_impl.cc

Index: chrome/browser/browser_process_impl.cc
diff --git a/chrome/browser/browser_process_impl.cc b/chrome/browser/browser_process_impl.cc
index e2601849a7792c07b92499cda5717a2a81b4b9e2..1bcb9e72a0923e186880f3f0bc99ef68fd48a594 100644
--- a/chrome/browser/browser_process_impl.cc
+++ b/chrome/browser/browser_process_impl.cc
@@ -13,6 +13,7 @@
 #include "base/path_service.h"
 #include "base/task.h"
 #include "base/thread.h"
+#include "base/thread_restrictions.h"
 #include "base/waitable_event.h"
 #include "chrome/browser/appcache/chrome_appcache_service.h"
 #include "chrome/browser/automation/automation_provider_list.h"
@@ -250,6 +251,12 @@ unsigned int BrowserProcessImpl::ReleaseModule() {
   DCHECK_NE(0u, module_ref_count_);
   module_ref_count_--;
   if (0 == module_ref_count_) {
+    // Allow UI and IO threads to do blocking IO on shutdown, since we do a lot
+    // of it on shutdown for valid reasons.
+    base::ThreadRestrictions::SetIOAllowed(true);
+    io_thread()->message_loop()->PostTask(
+        FROM_HERE,
+        NewRunnableFunction(&base::ThreadRestrictions::SetIOAllowed, true));
     MessageLoop::current()->PostTask(
         FROM_HERE, NewRunnableFunction(DidEndMainMessageLoop));
     MessageLoop::current()->Quit();