Add trace event for fonts enumeration in content

This CL is adding a trace event to observe the side-effects of fonts
enumeration in content.

This API is used font_settings_api.
See: chrome/browser/extensions/api/font_settings/font_settings_api.cc

Bug: 944227
Change-Id: Id6af649c3abc2f6806abf9d11a5df17e91ce915c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604313Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659194}

content/common/font_list_win.cc

@@ -11,6 +11,7 @@
 #include <utility>
 
 #include "base/strings/string16.h"
+#include "base/trace_event/trace_event.h"
 #include "base/values.h"
 
 namespace content {
@@ -32,6 +33,7 @@ static int CALLBACK EnumFontFamExProc(ENUMLOGFONTEXW* logical_font,
 }
 
 std::unique_ptr<base::ListValue> GetFontList_SlowBlocking() {
+  TRACE_EVENT0("fonts", "GetFontList_SlowBlocking");
   std::set<base::string16> font_names;
 
   LOGFONTW logfont;