Move core/platform/network to platform/network

Now that we've cleaned up this dependencies, we can finish moving the
networking code into blink_platform.dll.

R=eseidel@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/29123004

git-svn-id: svn://svn.chromium.org/blink/trunk@160019 bbb929c8-8fbe-4397-9dbb-9b2b20218538