Refactoring: Move m_mayDisplaySeamlesslyWithParent down to Document

The member is used only in Document, thus no reason to
stay in SecurityContext.

TEST=none
BUG=none
R=haraken@chromium.org, abarth, haraken, hayato

Review URL: https://codereview.chromium.org/27615003

git-svn-id: svn://svn.chromium.org/blink/trunk@159829 bbb929c8-8fbe-4397-9dbb-9b2b20218538