Remove FrameLoader::m_needsClear

Based on when it is reset and checked, its only purpose is to prevent
clearing a nonexistent document during frame initialization.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22972006

git-svn-id: svn://svn.chromium.org/blink/trunk@156491 bbb929c8-8fbe-4397-9dbb-9b2b20218538