Rename webkit to blink in trace event categories.

This CL updates the trace events to use 'blink' as the category instead
of 'webkit'.

BUG=386847

Review URL: https://codereview.chromium.org/344723002

git-svn-id: svn://svn.chromium.org/blink/trunk@176711 bbb929c8-8fbe-4397-9dbb-9b2b20218538