Add a LICENSE file cloned from Chromium's repo.

This is intended to clarify that Blink is licensed under the same terms
as the other Chromium repos, except where otherwise noted.

R=dannyb@google.com, eseidel@chromium.org
BUG=336943

Review URL: https://codereview.chromium.org/144313004

git-svn-id: svn://svn.chromium.org/blink/trunk@165583 bbb929c8-8fbe-4397-9dbb-9b2b20218538