DevTools: don't save pageScaleFactor to the HistoryItem while device emulation is enabled.

BUG=314105

Review URL: https://codereview.chromium.org/55903003

git-svn-id: svn://svn.chromium.org/blink/trunk@161198 bbb929c8-8fbe-4397-9dbb-9b2b20218538