Move m_containsPlugins from FrameLoader to Document

It's a property of the Document, not the Frame, and should never persist through
navigation.

BUG=none

Review URL: https://codereview.chromium.org/48283003

git-svn-id: svn://svn.chromium.org/blink/trunk@161207 bbb929c8-8fbe-4397-9dbb-9b2b20218538