Switch from WebDocument::insertUserStyleSheet() to insertStyleSheet().

A UserStyleLevel parameter is no longer used in insertUserStyleSheet()
and insertUserStyleSheet() calls just insertStyleSheet() again.

blink side: https://codereview.chromium.org/143203006

Review URL: https://codereview.chromium.org/154113008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250982 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250982