Fix "repeats" value evaluation for timer install event in timeline

Made repeats be the same as !singleShot.
Drive-by: Repeats and Timeout properties are only available for TimerInstall event.
Drive-by: fix GCEvent delta calculation.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/303443006

git-svn-id: svn://svn.chromium.org/blink/trunk@174807 bbb929c8-8fbe-4397-9dbb-9b2b20218538