Simplify URL setSearch

No change in behavior. Handling of leading '?' is handled by KURL and the escaping of '#' is handled by GURL.

BUG=None

Review URL: https://codereview.chromium.org/27217004

git-svn-id: svn://svn.chromium.org/blink/trunk@159710 bbb929c8-8fbe-4397-9dbb-9b2b20218538