Add support for a 'default' target to GN.
Add support for a 'default' target to GN.

Many build systems (including make and ninja) have the
concept of a "default" target that should be built if
you don't specify something explicitly to build: in
make, it's the first target found in the Makefile; in ninja
it is the target declared with the "default" directive, and
if that isn't found, the first target in the ninja file.

This CL adds the ability to set the "default" target
in the GN file; if it isn't set, we use GN's prior behavior,
which is to use "all" as the default target, and build
everything.

R=brettw@chromium.org
BUG=440644

Review URL: https://codereview.chromium.org/784413004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#307770}

fujunwei/chromium-crosswalk,Fireblend/chromium-crosswalk,dushu1203/chromium.src,Chilledheart/chromium,chuan9/chromium-crosswalk,krieger-od/nwjs_chromium.src,Just-D/chromium-1,fujunwei/chromium-crosswalk,dednal/chromium.src,markYoungH/chromium.src,axinging/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Chilledheart/chromium,krieger-od/nwjs_chromium.src,ltilve/chromium,PeterWangIntel/chromium-crosswalk,Chilledheart/chromium,Jonekee/chromium.src,Chilledheart/chromium,krieger-od/nwjs_chromium.src,hgl888/chromium-crosswalk,dednal/chromium.src,jaruba/chromium.src,ltilve/chromium,dednal/chromium.src,mohamed--abdel-maksoud/chromium.src,TheTypoMaster/chromium-crosswalk,Jonekee/chromium.src,jaruba/chromium.src,Jonekee/chromium.src,Jonekee/chromium.src,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,ltilve/chromium,axinging/chromium-crosswalk,M4sse/chromium.src,Pluto-tv/chromium-crosswalk,M4sse/chromium.src,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,dushu1203/chromium.src,dushu1203/chromium.src,mohamed--abdel-maksoud/chromium.src,chuan9/chromium-crosswalk,Just-D/chromium-1,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,M4sse/chromium.src,markYoungH/chromium.src,Fireblend/chromium-crosswalk,dushu1203/chromium.src,mohamed--abdel-maksoud/chromium.src,markYoungH/chromium.src,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,jaruba/chromium.src,chuan9/chromium-crosswalk,krieger-od/nwjs_chromium.src,ltilve/chromium,dushu1203/chromium.src,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,dednal/chromium.src,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,M4sse/chromium.src,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,ltilve/chromium,Jonekee/chromium.src,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,dushu1203/chromium.src,fujunwei/chromium-crosswalk,hgl888/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,M4sse/chromium.src,mohamed--abdel-maksoud/chromium.src,chuan9/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,jaruba/chromium.src,axinging/chromium-crosswalk,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,M4sse/chromium.src,Chilledheart/chromium,markYoungH/chromium.src,Pluto-tv/chromium-crosswalk,krieger-od/nwjs_chromium.src,Just-D/chromium-1,M4sse/chromium.src,chuan9/chromium-crosswalk,Jonekee/chromium.src,Chilledheart/chromium,dushu1203/chromium.src,jaruba/chromium.src,Pluto-tv/chromium-crosswalk,markYoungH/chromium.src,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,jaruba/chromium.src,Chilledheart/chromium,Just-D/chromium-1,Just-D/chromium-1,dednal/chromium.src,Fireblend/chromium-crosswalk,markYoungH/chromium.src,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,dednal/chromium.src,PeterWangIntel/chromium-crosswalk,dednal/chromium.src,Fireblend/chromium-crosswalk,jaruba/chromium.src,M4sse/chromium.src,Pluto-tv/chromium-crosswalk,Jonekee/chromium.src,ltilve/chromium,M4sse/chromium.src,ltilve/chromium,dushu1203/chromium.src,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,fujunwei/chromium-crosswalk,krieger-od/nwjs_chromium.src,markYoungH/chromium.src,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,markYoungH/chromium.src,Fireblend/chromium-crosswalk,krieger-od/nwjs_chromium.src,ltilve/chromium,Jonekee/chromium.src,fujunwei/chromium-crosswalk,dednal/chromium.src,jaruba/chromium.src,dushu1203/chromium.src,dushu1203/chromium.src,M4sse/chromium.src,dednal/chromium.src,axinging/chromium-crosswalk,krieger-od/nwjs_chromium.src,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,markYoungH/chromium.src,chuan9/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,markYoungH/chromium.src,fujunwei/chromium-crosswalk,M4sse/chromium.src,axinging/chromium-crosswalk,Jonekee/chromium.src,mohamed--abdel-maksoud/chromium.src,markYoungH/chromium.src,jaruba/chromium.src,Jonekee/chromium.src,jaruba/chromium.src,mohamed--abdel-maksoud/chromium.src,krieger-od/nwjs_chromium.src,dushu1203/chromium.src,fujunwei/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,jaruba/chromium.src,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,krieger-od/nwjs_chromium.src,krieger-od/nwjs_chromium.src,Fireblend/chromium-crosswalk,Jonekee/chromium.src,fujunwei/chromium-crosswalk,ltilve/chromium,dednal/chromium.src,dednal/chromium.src,Just-D/chromium-1