Make event names of TRACE_EVENT_SAMPLING_STATE macros more descriptive

"Execution" => "V8Execution"
"MinorGC" => "V8MinorGC" and "DOMMinorGC"
"MajorGC" => "V8MajorGC" and "DOMMajorGC"
etc

Review URL: https://codereview.chromium.org/131203009

git-svn-id: svn://svn.chromium.org/blink/trunk@165076 bbb929c8-8fbe-4397-9dbb-9b2b20218538