Note the overhead of no-parameter TraceEvent methods

The overhead of no-parameter TraceEvent methods is at ms or sub-ms
order. Note the fact in javadoc.

BTW, removed unnecessary references to #begin().

BUG=340872
NOTRY=true

Review URL: https://codereview.chromium.org/155903004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249067 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249067