Rename PageGroupLoadDeferrer to ScopedPageLoadDeferrer

In Chrome there is only one page group consisting of all normal
pages, and we are in the process of getting rid of PageGroup.

The new name makes it clear that the guard is scoped, and the ctor
is changed so that it optionally takes a page as argument which
will not be deferred.

Updated all call sites and simplified them a bit. The class is
also simplified slightly given the new argument.

BUG=288249

Review URL: https://codereview.chromium.org/172513002

git-svn-id: svn://svn.chromium.org/blink/trunk@167457 bbb929c8-8fbe-4397-9dbb-9b2b20218538