tools/gn: Fix for running ./bootstrap.py script.
tools/gn: Fix for running ./bootstrap.py script.

Otherwise you can get an error like this:

Traceback (most recent call last):
File "./bootstrap.py", line 413, in <module>
sys.exit(main(sys.argv[1:]))
File "./bootstrap.py", line 86, in main
build_gn_with_gn(temp_gn, build_rel, options)
File "./bootstrap.py", line 408, in build_gn_with_gn
if not debug:
NameError: global name 'debug' is not defined

BUG=None
TEST=cd tools/gn/bootstrap && ./bootstrap.py
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/834063003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#310999}

PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,ltilve/chromium,ltilve/chromium,ltilve/chromium,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,fujunwei/chromium-crosswalk,Fireblend/chromium-crosswalk,Just-D/chromium-1,fujunwei/chromium-crosswalk,fujunwei/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ltilve/chromium,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ltilve/chromium,Chilledheart/chromium,hgl888/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,fujunwei/chromium-crosswalk,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,ltilve/chromium,Chilledheart/chromium,chuan9/chromium-crosswalk,ltilve/chromium,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,ltilve/chromium,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk