Introduce Page::allPages() as an accessor to all current pages

This allows accessing all pages without relying on the page groups.

Use it inside Page.cpp.

BUG=288249

Review URL: https://codereview.chromium.org/132883012

git-svn-id: svn://svn.chromium.org/blink/trunk@166020 bbb929c8-8fbe-4397-9dbb-9b2b20218538