Add top-level trace markers for each benchmark

Also add new paragraph about using systrace correctly

Docs-Preview: https://skia.org/?cl=41502
Change-Id: I114c14cc2e87a8b72aec46d8c354d3ea877a41ab
Reviewed-on: https://skia-review.googlesource.com/41502
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index bd23c3e..d69db82 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -46,6 +46,7 @@
 #include "SkSurface.h"
 #include "SkTaskGroup.h"
 #include "SkThreadUtils.h"
+#include "SkTraceEvent.h"
 #include "Stats.h"
 #include "ThermalManager.h"
 #include "ios_utils.h"
@@ -1272,6 +1273,8 @@
                 }
             }
 
+            TRACE_EVENT1("skia", bench->getUniqueName(), "config", config);
+
             target->setup();
             bench->perCanvasPreDraw(canvas);