Remove TRACE_EVENT_PHASE_INSTANT_WITH_SCOPE

This change unifies instant event phase name between Chromium and Blink. Now it is 'I' in both projects.

BUG=None
R=loislo@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/408443002

git-svn-id: svn://svn.chromium.org/blink/trunk@178641 bbb929c8-8fbe-4397-9dbb-9b2b20218538