gyp: use a set() in DeepDependencies for less O(n^2). 

Another O(n^2) over the number of targets which used about half a second 
for a 1000 target system (chromium) on a reasonable computer. 

BUG=chromium:362075
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/235193002/
Patch from Daniel Bratell <bratell@opera.com>!


git-svn-id: http://gyp.googlecode.com/svn/trunk@1899 78cadc50-ecff-11dd-a971-7dbc132099af
1 file changed