Move NetworkStateNotifier into core/page

This class used to be in platform because other WebKit ports actually provided
implementations as part of platform. But in Blink, the state is always passed
in from the embedder, leaving this code platform-agnostic.

As part of the move, also fixed the thread-safety of the class. It was assuming
that since the value was "just a bool" it would be safe to access from
multiple threads without a lock. Now uses a Mutex to serialize access.

R=abarth@chromium.org, abarth

Review URL: https://codereview.chromium.org/25762003

git-svn-id: svn://svn.chromium.org/blink/trunk@159033 bbb929c8-8fbe-4397-9dbb-9b2b20218538