XMLTreeViewer should be created only in the XML viewer mode

Moving XMLTreeViewer::hasNoStyleInformation to be static inline method in XMLDocumentParser.cpp as
it should not be the responsibility of XMLTreeViewer.

R=abarth

Review URL: https://chromiumcodereview.appspot.com/22150003

git-svn-id: svn://svn.chromium.org/blink/trunk@155521 bbb929c8-8fbe-4397-9dbb-9b2b20218538