Rename Blink to blink in Trace Events.

We currently have some trace events name 'Blink' and some named 'blink'. This
CL renames all of the 'Blink' trace events to be 'blink' for consistency.

This makes the chrome://tracing UI less confusing as you no longer have to
realize there are both 'Blink' and 'blink' categories to be enabled.

Review URL: https://codereview.chromium.org/353503002

git-svn-id: svn://svn.chromium.org/blink/trunk@176931 bbb929c8-8fbe-4397-9dbb-9b2b20218538