Skip running GYP if GYP_CHROMIUM_NO_ACTION is set in environment

This is how GYP is skipped on the GN bots. If GYP runs, it will currently
produce files in out/{Debug,Releaese} that makes GN fail.

BUG=601140
TBR=fbarchard@chromium.org

Review URL: https://codereview.chromium.org/1874653003 .

gyp_libyuv

@@ -37,6 +37,10 @@ def GetSupplementalFiles():
 if __name__ == '__main__':
   args = sys.argv[1:]
 
+  if int(os.environ.get('GYP_CHROMIUM_NO_ACTION', 0)):
+    print 'Skipping gyp_libyuv due to GYP_CHROMIUM_NO_ACTION env var.'
+    sys.exit(0)
+
   # This could give false positives since it doesn't actually do real option
   # parsing.  Oh well.
   gyp_file_specified = False