DevTools: only events with phase "I" should be recognized as instant events

After r178641 we don't use "i" for instant events in Blink. Support for "i" is removed.

BUG=None
R=alph@chromium.org

Review URL: https://codereview.chromium.org/493083002

git-svn-id: svn://svn.chromium.org/blink/trunk@180713 bbb929c8-8fbe-4397-9dbb-9b2b20218538