Un-inline KURL's default constructor to avoid bloating

Un-inline KURL's default constructor to avoid bloating. KURL has non-POD data
members and its constructor is thus non-trivial.

This reduces the release binary size by ~17Kb.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/307193002

git-svn-id: svn://svn.chromium.org/blink/trunk@175216 bbb929c8-8fbe-4397-9dbb-9b2b20218538