Trace event micro-optimization: create "toplevel" category.

Some traces show that MessageLoop::RunTask is a hotspot.
It eventually calls into GetCategoryGroupEnabledInternal.
By making "toplevel" a built-in category, it saves a few strcmp during RunTask.

BUG=

Review URL: https://codereview.chromium.org/143573005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250671 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250671