Clean up the way we determine which env vars to pass to content_shell.

We were passing more variables across than necessary, duplicating some
work, and things were generally unclear. This patch consolidates most
of the logic and makes things clearer.

R=ojan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/22978015

git-svn-id: svn://svn.chromium.org/blink/trunk@156355 bbb929c8-8fbe-4397-9dbb-9b2b20218538