Reland "Implement worler worlds for workers" (part 2)

This CL is a part of https://codereview.chromium.org/151443002.
Since landing https://codereview.chromium.org/151443002 caused some crashes
(https://code.google.com/p/chromium/issues/detail?id=338122) which I cannot
reproduce/understand, I'm landing the CL very incrementally.
This is the second part of the incremental landing.

BUG=339800

Review URL: https://codereview.chromium.org/150283007

git-svn-id: svn://svn.chromium.org/blink/trunk@166706 bbb929c8-8fbe-4397-9dbb-9b2b20218538