Move allowPlugins() and m_containsPlugins from SubframeLoader to FrameLoader

One of several steps in removing SubframeLoader, which only has a vague purpose
and can (apart from this patch) be nicely rolled into HTMLFrameOwnerElement and
its subclasses.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/20852002

git-svn-id: svn://svn.chromium.org/blink/trunk@155051 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Merge SubframeLoader::requestFrame into its only caller, HTMLFrameElementBase::openURL

BUG=none

Review URL: https://chromiumcodereview.appspot.com/20860002

git-svn-id: svn://svn.chromium.org/blink/trunk@155052 bbb929c8-8fbe-4397-9dbb-9b2b20218538