Move SocketStream to platform/network

This CL creates the platform/network directory and moves the first few files
from core/platform/network.

R=eseidel@chromium.org, eseidel
BUG=297477

Review URL: https://codereview.chromium.org/25722006

git-svn-id: svn://svn.chromium.org/blink/trunk@158840 bbb929c8-8fbe-4397-9dbb-9b2b20218538