Add missing comma in the GN build.

GN currently has a bug where commas are not required in argument lists. This fix is in preparation for pushing a GN with that bug fixed.

BUG=368019
TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/251213003

git-svn-id: svn://svn.chromium.org/blink/trunk@172828 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Measure Event.returnValue usage in more detail

As suggested by Boris Zbarsky:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=25002#c2

BUG=none

Review URL: https://codereview.chromium.org/256773006

git-svn-id: svn://svn.chromium.org/blink/trunk@172829 bbb929c8-8fbe-4397-9dbb-9b2b20218538