This is a precursor for implementing better checks when setting document.domain.

This folds weborigin/ back under platform/ to allow it to call Platform APIs. Weborigin was originally split off to allow it to be called in other places; turns out this was not necessary.

The tests in the weborigin_unittests target are now folded back into blink_platform_unittests. The weborigin_unittests will no longer exist.

BUG=307407

Review URL: https://codereview.chromium.org/54053006

git-svn-id: svn://svn.chromium.org/blink/trunk@161946 bbb929c8-8fbe-4397-9dbb-9b2b20218538