[DevTools] Supply location instead of event to InspectorFrontendHost.showContextMenu method.

This change allows to show asynchronous menus.

BUG=none

Review URL: https://codereview.chromium.org/362253003

git-svn-id: svn://svn.chromium.org/blink/trunk@177537 bbb929c8-8fbe-4397-9dbb-9b2b20218538