Background tracing: Ensure config is present on histogram trigger IPCs

R=ssid@chromium.org

Bug: 788306
Change-Id: I2e81ab0c1836aebeb2ed36e02c38e39309557057
Reviewed-on: https://chromium-review.googlesource.com/793897Reviewed-by: Siddhartha S <ssid@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522492}

content/browser/tracing/background_tracing_manager_impl.cc

@@ -323,6 +323,9 @@ void BackgroundTracingManagerImpl::OnHistogramTrigger(
     return;
   }
 
+  if (!config_)
+    return;
+
   for (const auto& rule : config_->rules()) {
     if (rule->ShouldTriggerNamedEvent(histogram_name))
       OnRuleTriggered(rule.get(), StartedFinalizingCallback());