DevTools: Fix broken "Store as Global Variable" while on pause.

We should always evaluate "this" on global object, not on a call frame, where
"this" would resolve to a receiver object of the function call frame.

R=vsevik, loislo

Review URL: https://codereview.chromium.org/646983009

git-svn-id: svn://svn.chromium.org/blink/trunk@184463 bbb929c8-8fbe-4397-9dbb-9b2b20218538