New option for arithmetic mode

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 95 matching lines @@
       'effects/SkDisplacementMapEffect.h',
       'effects/SkPixelXorXfermode.h',
       'effects/SkColorMatrixFilter.h',
       'effects/SkColorMatrix.h',
       'effects/SkBlurDrawLooper.h',
       'effects/SkColorFilterImageFilter.h',
       'effects/SkLayerDrawLooper.h',
       'effects/SkLightingImageFilter.h',
       'effects/SkDropShadowImageFilter.h',
       'effects/SkMatrixConvolutionImageFilter.h',
+      'effects/SkValidateImageFilter.h',
       'utils/win/SkAutoCoInitialize.h',
       'utils/win/SkHRESULT.h',
       'utils/win/SkIStream.h',
       'utils/win/SkTScopedComPtr.h',
       'utils/SkBoundaryPatch.h',
       'utils/SkUnitMappers.h',
       'utils/SkPictureUtils.h',
       'utils/SkRandom.h',
       'utils/SkMeshUtils.h',
       'utils/SkCullPoints.h',
@@ skipping 163 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',
     ],
   },
 }