[trace] Add 'benchmark' category to the FCP trace.

Add the 'benchmark' category to the trace-event that reports the
FirstContentfulPaint event, so that the trace gets included for all
benchmarks (without all benchmarks having to enable all traces of
'loading' category).

BUG=1158862

Change-Id: Id1bd528f735de625002a1210ab75896faaece99d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593479
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837387}

third_party/blink/renderer/core/paint/paint_timing.cc

@@ -336,7 +336,8 @@ void PaintTiming::SetFirstPaintPresentation(base::TimeTicks stamp) {
 
 void PaintTiming::SetFirstContentfulPaintPresentation(base::TimeTicks stamp) {
   DCHECK(first_contentful_paint_presentation_.is_null());
-  TRACE_EVENT_INSTANT_WITH_TIMESTAMP0("loading", "FirstContentfulPaint",
+  TRACE_EVENT_INSTANT_WITH_TIMESTAMP0("benchmark,loading",
+                                      "FirstContentfulPaint",
                                       TRACE_EVENT_SCOPE_GLOBAL, stamp);
   first_contentful_paint_presentation_ = stamp;
   probe::PaintTiming(