Change default generators on windows to include msvs-ninja rather than msvs

BUG=skia:2439
R=epoger@google.com
TBR=epoger@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/317903002
diff --git a/gyp_skia b/gyp_skia
index e04fde0..040253e 100755
--- a/gyp_skia
+++ b/gyp_skia
@@ -85,9 +85,9 @@
     if sys.platform.startswith('darwin'):
       default_gyp_generators = 'ninja,xcode'
     elif sys.platform.startswith('win'):
-      default_gyp_generators = 'ninja,msvs'
+      default_gyp_generators = 'ninja,msvs-ninja'
     elif sys.platform.startswith('cygwin'):
-      default_gyp_generators = 'ninja,msvs'
+      default_gyp_generators = 'ninja,msvs-ninja'
     else:
       default_gyp_generators = 'ninja'
     os.environ[ENVVAR_GYP_GENERATORS] = default_gyp_generators