Add trace event for tracking triggered rules and values

This CL is adding an instant event to put the histogram value into the trace.
The UMA histogram is not part of the trace and it is making it hard to
figure out what happened in a reported trace.

The instant message is also giving an hint at which time the histogram
triggered the corresponding rule.

R=oysteine@chromium.org

Change-Id: I34fdf84c5b6d54ec4b8cd35dff8799d786a85334
Reviewed-on: https://chromium-review.googlesource.com/1151965Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578460}

content/browser/tracing/background_tracing_rule.cc

@@ -267,6 +267,12 @@ class HistogramRule
       return;
     }
 
+    // Add the histogram name and its corresponding value to the trace.
+    TRACE_EVENT_INSTANT2("toplevel",
+                         "BackgroundTracingRule::OnHistogramTrigger",
+                         TRACE_EVENT_SCOPE_THREAD, "histogram_name",
+                         histogram_name, "value", actual_value);
+
     OnHistogramTrigger(histogram_name);
   }