Get rid of PageGroup

The page group is a leftover concept from WebKit.

We now have a new getter for ordinary pages (all pages, including
special ones as SVG, inspector overlay etc can be accessed already).

BUG=288249

Review URL: https://codereview.chromium.org/180983003

git-svn-id: svn://svn.chromium.org/blink/trunk@168040 bbb929c8-8fbe-4397-9dbb-9b2b20218538