Break core/platform/network dependency on core

This CL breaks the last dependency from core/platform/network to the rest of
core. I've moved FormDataList back to core and made it responsible for building
FormData objects out of Blobs (and other items). That means FormData is not
just data, which makes sense.

After this CL and https://codereview.chromium.org/26283005/ land, we'll be able
to move core/platform/network into blink_platform.dll.

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

Review URL: https://codereview.chromium.org/27537012

git-svn-id: svn://svn.chromium.org/blink/trunk@159951 bbb929c8-8fbe-4397-9dbb-9b2b20218538