Make --undefok a little louder now that we're using it all the time.

BUG=skia:

Review URL: https://codereview.chromium.org/1073483002
diff --git a/tools/flags/SkCommandLineFlags.cpp b/tools/flags/SkCommandLineFlags.cpp
index 6f1491b..8929c38 100644
--- a/tools/flags/SkCommandLineFlags.cpp
+++ b/tools/flags/SkCommandLineFlags.cpp
@@ -313,8 +313,10 @@
                         i++;  // skip YES
                 } else
 #endif
-                if (!FLAGS_undefok) {
-                    SkDebugf("Got unknown flag \"%s\". Exiting.\n", argv[i]);
+                if (FLAGS_undefok) {
+                    SkDebugf("FYI: ignoring unknown flag '%s'.\n", argv[i]);
+                } else {
+                    SkDebugf("Got unknown flag '%s'. Exiting.\n", argv[i]);
                     exit(-1);
                 }
             }