net: Make HttpNetworkLayer subscribe to power events on Windows.

HttpNetworkLayer unintentionally stopped subscribing for power events 1 year
and 6 months ago (see r86241).

This is not a big problem in practice though since URLRequestJob already
subscribes to power events. The socket pool is also invalidated when the
network change notifier fires a 'no connection' event after a suspend on
desktop.

This makes HttpNetworkLayer subscribe to power events on Windows which is the
platform that initially needed it.

BUG=318326
TEST=see bug
TBR=pauljensen@chromium.org

This CL is just a rebased version of https://codereview.chromium.org/11572039/
(which never landed but was reviewed).

Review URL: https://codereview.chromium.org/89583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237508 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237508