change default build (in "make" wrapper) to ninja on all platforms

gyp_skia

Index: gyp_skia
diff --git a/gyp_skia b/gyp_skia
index 1ba6fc78e86360c4dc712e7b55a480899fa5d562..7a4502684f7a1a477fecabe451a55525764f939a 100755
--- a/gyp_skia
+++ b/gyp_skia
@@ -84,13 +84,13 @@ if __name__ == '__main__':
     print ('%s environment variable not set, using default' %
            ENVVAR_GYP_GENERATORS)
     if sys.platform.startswith('darwin'):
-      default_gyp_generators = 'xcode'
+      default_gyp_generators = 'ninja,xcode'
     elif sys.platform.startswith('win'):
-      default_gyp_generators = 'msvs'
+      default_gyp_generators = 'ninja,msvs'
     elif sys.platform.startswith('cygwin'):
-      default_gyp_generators = 'msvs'
+      default_gyp_generators = 'ninja,msvs'
     else:
-      default_gyp_generators = 'make'
+      default_gyp_generators = 'ninja'
     os.environ[ENVVAR_GYP_GENERATORS] = default_gyp_generators
   print '%s is "%s"' % (ENVVAR_GYP_GENERATORS, os.getenv(ENVVAR_GYP_GENERATORS))