Add a developer hint to an otherwise cryptic error when accidently registering...

Add a developer hint to an otherwise cryptic error when accidently registering a second RunLoop::Delegate on a thread.

While Runloop::Delegate isn't aware of MessageLoop/ScopedTaskEnvironment:
I find useful to add developer hints about the typical developer mistake
a DCHECK catches. This avoids multiple developers wasting time figuring
out the same thing. Started doing this in a few places in //base and this
is another instance recently surfaced by chrisha @
https://groups.google.com/a/google.com/d/topic/lucky-luke-dev/9_eZ6pnvo7Y/discussion

R=chrisha@chromium.org

Bug: Cryptic error when creating a ScopedTaskEnvironment over an
existing MessageLoop.

Change-Id: I427e0400b2a2b91ab9b175fa2427014bc8fc4406
Reviewed-on: https://chromium-review.googlesource.com/890174
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532402}

base/run_loop.cc

@@ -63,7 +63,10 @@ void RunLoop::RegisterDelegateForCurrentThread(Delegate* delegate) {
   DCHECK_CALLED_ON_VALID_THREAD(delegate->bound_thread_checker_);
 
   // There can only be one RunLoop::Delegate per thread.
-  DCHECK(!tls_delegate.Get().Get());
+  DCHECK(!tls_delegate.Get().Get())
+      << "Error: Multiple RunLoop::Delegates registered on the same thread.\n\n"
+         "Hint: You perhaps instantiated a second "
+         "MessageLoop/ScopedTaskEnvironment on a thread that already had one?";
   tls_delegate.Get().Set(delegate);
   delegate->bound_ = true;
 }