Move the injected style sheet code out of PageGroup

Create a new global InjectedStyleSheets object and move the
related code to it.

BUG=288249

Review URL: https://codereview.chromium.org/137623013

git-svn-id: svn://svn.chromium.org/blink/trunk@166601 bbb929c8-8fbe-4397-9dbb-9b2b20218538