Oilpan: move ScriptLoader to the heap.

      So as to be able to trace its PendingScript part object.

(Initially landed as r183909, relanded after r183984 fix.)

R=haraken
BUG=

Review URL: https://codereview.chromium.org/660233002

git-svn-id: svn://svn.chromium.org/blink/trunk@184013 bbb929c8-8fbe-4397-9dbb-9b2b20218538