Refactoring: Extract Document::processHttpEquivContentSecurityPolicy().

BUG=none
TEST=none
R=mkwst@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19958004

git-svn-id: svn://svn.chromium.org/blink/trunk@154729 bbb929c8-8fbe-4397-9dbb-9b2b20218538