remove SkDataSet, and just store a key/value in SkAnnotation

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 234 matching lines @@
       '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/SkImageFilterUtils.h',
       'core/SkLineClipper.h',
-      'core/SkDataSet.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',
@@ skipping 39 matching lines @@
       'lazy/SkLruImageCache.h',
     ],
   },
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: