Rename "blink_rendering" trace events to "blink"

This patch renames "blink_rendering" trace events to just "blink". The
separation caused confusion as people tracing with the blink category
would miss these events.

I can't imagine a case where these events would be useful on their own.

Review URL: https://codereview.chromium.org/454683002

git-svn-id: svn://svn.chromium.org/blink/trunk@179777 bbb929c8-8fbe-4397-9dbb-9b2b20218538