Added trace event to V8WindowShell::initializeIfNeeded. This is part of an effort to identify what contributes to the ~12.6% &quotuncategorized&quot time from the android top 25k experiment. For sites high in uncategorized time, this accounts for about 3.8% of the trace.

      BUG=

Review URL: https://codereview.chromium.org/26291004

git-svn-id: svn://svn.chromium.org/blink/trunk@159164 bbb929c8-8fbe-4397-9dbb-9b2b20218538