When changing the object constructor mode, always supperss microtasks

Otherwise, we might run arbitrary javascript that could try to construct
the wrong kind of objects

BUG=none
R=dcarney@chromium.org,adamk@chromium.org

Review URL: https://codereview.chromium.org/263143002

git-svn-id: svn://svn.chromium.org/blink/trunk@173303 bbb929c8-8fbe-4397-9dbb-9b2b20218538