DevTools: make context menu and inspect element work with non-1 pageScaleFactor.

BUG=314098
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/56053002

git-svn-id: svn://svn.chromium.org/blink/trunk@161179 bbb929c8-8fbe-4397-9dbb-9b2b20218538