Expose showModalDialog runtime flag via WebRuntimeFeatures

BUG=none
R=ojan@chromium.org,atwilson@chromium.org

Review URL: https://codereview.chromium.org/329873002

git-svn-id: svn://svn.chromium.org/blink/trunk@176037 bbb929c8-8fbe-4397-9dbb-9b2b20218538