Remove now-nonexistent SkCountdown.h from public_headers.

Seems to be breaking Chrome's GN build.

R=rmistry@google.com
TBR=rmistry
BUG=skia:

Author: senorblanco@chromium.org

Review URL: https://codereview.chromium.org/359183002
diff --git a/gyp/public_headers.gypi b/gyp/public_headers.gypi
index 7001d79..d6c7629 100644
--- a/gyp/public_headers.gypi
+++ b/gyp/public_headers.gypi
@@ -119,7 +119,6 @@
       'utils/SkCamera.h',
       'utils/SkLua.h',
       'utils/SkParsePaint.h',
-      'utils/SkCountdown.h',
       'utils/SkRunnable.h',
       'utils/SkParse.h',
       'utils/SkThreadPool.h',