SmartLock: Run the IO thread on mainloop instead of a real thread.

This fixes a crashing test (with DCHECK enabled) caused by the IO thread
having a different ID for each test. This conflicted with the constraint of
the NSSInitSingleton that its methods be called with thread-affinity (on
the same thread ID).

R=khorimoto@chromium.org

Bug: 828538
Test: EasyUnlockTpmKeyManagerTest.*
Change-Id: I7936ed9ad09884a47a898344a99977d3d5d013ac
Reviewed-on: https://chromium-review.googlesource.com/994071Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547863}

chrome/browser/chromeos/login/easy_unlock/easy_unlock_tpm_key_manager_unittest.cc

@@ -196,7 +196,7 @@ void RecordStringAndRunClosure(std::string* result,
 class EasyUnlockTpmKeyManagerTest : public testing::Test {
  public:
   EasyUnlockTpmKeyManagerTest()
-      : thread_bundle_(content::TestBrowserThreadBundle::REAL_IO_THREAD),
+      : thread_bundle_(content::TestBrowserThreadBundle::IO_MAINLOOP),
         user_manager_(new FakeChromeUserManager()),
         user_manager_enabler_(base::WrapUnique(user_manager_)),
         profile_manager_(TestingBrowserProcess::GetGlobal()) {}