DevTools: fix Ctrl-Plus/Ctrl-Minus shortcuts for zooming in hosted mode

As action system handles global shortcuts asynchronously, it
optimistically captures event to prevent the default browser action.

This behavior results in unfunctioning zoom in/out shortcuts in
hosted mode.

The patch adds a workaround which doesn't let the action system
a chance to step in and prevent default behavior for the zooming
shortcuts in hosted mode.

R=pfeldman

Review URL: https://codereview.chromium.org/670303002

git-svn-id: svn://svn.chromium.org/blink/trunk@184274 bbb929c8-8fbe-4397-9dbb-9b2b20218538