Fix uninitialized field in WindowFeatures

Initialize x, y, width, and height in WindowFeatures constructor to prevent
warnings about using uninitialized memory.

R=dcheng@chromium.org
BUG=342591
TEST=manual

Review URL: https://codereview.chromium.org/177633011

git-svn-id: svn://svn.chromium.org/blink/trunk@171963 bbb929c8-8fbe-4397-9dbb-9b2b20218538