Don't show "Saving...0%" status until saving actually started

This fixes the problem that heap snapshot status is changed to "Saving...0%" before file name is confirmed.

Drive-by: removing some unused fields.

BUG=None
R=eustas@chromium.org

Review URL: https://codereview.chromium.org/23717025

git-svn-id: svn://svn.chromium.org/blink/trunk@157110 bbb929c8-8fbe-4397-9dbb-9b2b20218538