move SkChecksum and SkEmptyShader to private

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 204 matching lines @@
       'core/SkBlitRow.h',
       'core/SkGeometry.h',
       'core/SkStrokeRec.h',
       'core/SkImageDecoder.h',
       'core/SkTime.h',
       'core/SkPathMeasure.h',
       'core/SkMaskFilter.h',
       'core/SkFlate.h',
       'core/SkTDArray.h',
       'core/SkAnnotation.h',
-      'core/SkChecksum.h',
       'core/SkMath.h',
       'core/SkDrawLooper.h',
       'core/SkFlattenableBuffers.h',
       'core/SkTemplates.h',
       'core/SkMask.h',
       'core/SkMallocPixelRef.h',
       'core/SkWeakRefCnt.h',
       'core/SkTypes.h',
       'core/SkThread.h',
       'core/SkData.h',
       'core/SkPoint.h',
       'core/SkColorShader.h',
       'core/SkChunkAlloc.h',
       'core/SkUnPreMultiply.h',
       'core/SkReader32.h',
       'core/SkDevice.h',
       'core/SkImageFilter.h',
       'core/SkAdvancedTypefaceMetrics.h',
       'core/SkTInternalLList.h',
       'core/SkTArray.h',
       'core/SkStringUtils.h',
       'core/SkPreConfig.h',
       'core/SkLineClipper.h',
       'core/SkPathEffect.h',
       'core/SkString.h',
       'core/SkPixelRef.h',
       'core/SkSize.h',
-      'core/SkEmptyShader.h',
       'core/SkSurface.h',
       'core/SkPostConfig.h',
       'core/SkShader.h',
       'core/SkWriter32.h',
       'core/SkError.h',
       'core/SkPath.h',
       'core/SkFlattenable.h',
       'core/SkTSearch.h',
       'core/SkRect.h',
       'pathops/SkPathOps.h',
@@ skipping 22 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',
     ],
   },
 }