Add net_unittests to the GN build.

BUG=
R=rsleevi@chromium.org

Review URL: https://codereview.chromium.org/311193004

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@275186 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
Move about://-related constants from //content to //url

kAboutScheme and kAboutBlankURL are used in code shared by iOS, and are general
enough to live in //url instead of //content.

TBR=yoz,benm

Review URL: https://codereview.chromium.org/325443002

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@275988 4ff67af0-8c30-449e-8e8b-ad334ec8d88c