win: Use ninja by default

build/gyp_chromium

Index: build/gyp_chromium
diff --git a/build/gyp_chromium b/build/gyp_chromium
index 4eafdd541ed71cb9cd1caa434105d81bf3daa6a8..50240aaec0032685c7fe0bd1104c71097cfee269 100755
--- a/build/gyp_chromium
+++ b/build/gyp_chromium
@@ -290,13 +290,16 @@ if __name__ == '__main__':
   if sys.platform not in ('darwin',):
     args.append('--no-circular-check')
 
-  # Default to ninja on linux, but only if no generator has explicitly been set.
+  # Default to ninja on linux and windows, but only if no generator has
+  # explicitly been set.
   # Also default to ninja on mac, but only when not building chrome/ios.
   # . -f / --format has precedence over the env var, no need to check for it
   # . set the env var only if it hasn't been set yet
   # . chromium.gyp_env has been applied to os.environ at this point already
   if sys.platform.startswith('linux') and not os.environ.get('GYP_GENERATORS'):
     os.environ['GYP_GENERATORS'] = 'ninja'
+  if sys.platform.startswith('win') and not os.environ.get('GYP_GENERATORS'):
+    os.environ['GYP_GENERATORS'] = 'ninja'
   elif sys.platform == 'darwin' and not os.environ.get('GYP_GENERATORS') and \
       not 'OS=ios' in os.environ.get('GYP_DEFINES', []):
     os.environ['GYP_GENERATORS'] = 'ninja'