DevTools: handle zoom shortcuts in hosted mode on Mac OS X

The Ctrl+/Ctrl- shortcuts are already supported in hosted mode on linux
for some time. This patch adds the same supoprt for Cmd+/Cmd- shortcuts
on mac in hosted mode.

TBR=pfeldman

Review URL: https://codereview.chromium.org/702583003

git-svn-id: svn://svn.chromium.org/blink/trunk@184867 bbb929c8-8fbe-4397-9dbb-9b2b20218538