Make MergeLists much faster. 
Make MergeLists much faster. 

Previously, we tested membership in a list in a loop. This is very slow. So 
optimize the common case and test membership using a set instead. 

TEST=gyptest.py -a

Patch from T-Rungl Luu <viettrungluu@chromium.org>
Review URL: http://codereview.chromium.org/7375018/


old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google