Add a WebCore directory to the python path so we can use pymod_do_main

Add a WebCore directory to the python path so we can use pymod_do_main
in Webore.gyp.

In https://bugs.webkit.org/show_bug.cgi?id=106604 , I'm adding a script
for generating idl dependencies. To make it run faster, I'm using
pymod_do_main.

BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=176437

[tony, 2013-01-11 18:12:35]

On my machine with the patch on the webkit bug, the change to time to run
gyp_chromium is an increase of about 200ms with GYP_PARALLEL on or off. 
However, this saves over a minute if it avoids regenerating all the idl files.

[Nico, 2013-01-11 20:51:50]

lgtm!