Remove now-nonexistent SkCountdown.h from public_headers.

gyp/public_headers.gypi

 # Include this gypi to include all public header files that exist in the
 # include directory.
 #
 # The list is computed by running 'find include -name *.h' in the root dir of
 # the project.
 #
 {
   'variables': {
     'header_filenames': [
       'pdf/SkPDFDevice.h',
@@ skipping 101 matching lines @@
       'utils/win/SkIStream.h',
       'utils/win/SkTScopedComPtr.h',
       'utils/SkBoundaryPatch.h',
       'utils/SkPictureUtils.h',
       'utils/SkRandom.h',
       'utils/SkMeshUtils.h',
       'utils/SkCullPoints.h',
       'utils/SkCamera.h',
       'utils/SkLua.h',
       'utils/SkParsePaint.h',
-      'utils/SkCountdown.h',
       'utils/SkRunnable.h',
       'utils/SkParse.h',
       'utils/SkThreadPool.h',
       'utils/SkMatrix44.h',
       'utils/SkInterpolator.h',
       'utils/SkWGL.h',
       'utils/SkDumpCanvas.h',
       'utils/SkRTConf.h',
       'utils/SkCubicInterval.h',
       'utils/SkLuaCanvas.h',
@@ skipping 146 matching lines @@
       'views/unix/XkeysToSkKeys.h',
       'views/SkKey.h',
       'views/SkView.h',
       'views/SkOSMenu.h',
       'views/SkOSWindow_Unix.h',
       'views/SkWidget.h',
       'views/SkOSWindow_Win.h',
     ],
   },
 }