Rename V8 to v8 in trace events.

We used both V8 and v8 in trace event category names. This makes things
confusing when you start tracing as to which one to use. This CL migrates
all V8 categories to be v8.

Review URL: https://codereview.chromium.org/340393005

git-svn-id: svn://svn.chromium.org/blink/trunk@176884 bbb929c8-8fbe-4397-9dbb-9b2b20218538