Remove ScriptWrappable's destructor in Oilpan builds.

r177389 (http://crrev.com/362963005) made ScriptWrappable's destructor
empty in Oilpan builds. Take the next step and do not define it at all.

R=
BUG=340522

Review URL: https://codereview.chromium.org/369633003

git-svn-id: svn://svn.chromium.org/blink/trunk@177436 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Oilpan: build fix after r177372

TBR=sof@opera.com

Review URL: https://codereview.chromium.org/364833004

git-svn-id: svn://svn.chromium.org/blink/trunk@177437 bbb929c8-8fbe-4397-9dbb-9b2b20218538