Revert of Remove unused (by clients) SkUnitMapper (https://codereview.chromium.org/283273002/)

gyp/core.gypi

 # Include this gypi to include all 'core' files
 # The parent gyp/gypi file must define
 #       'skia_src_path'     e.g. skia/trunk/src
 #       'skia_include_path' e.g. skia/trunk/include
 #
 # The skia build defines these in common_variables.gypi
 #
 {
     'sources': [
         '<(skia_src_path)/core/SkAAClip.cpp',
@@ skipping 289 matching lines @@
         '<(skia_include_path)/core/SkTileGridPicture.h',
         '<(skia_include_path)/core/SkTRegistry.h',
         '<(skia_include_path)/core/SkTSearch.h',
         '<(skia_include_path)/core/SkTemplates.h',
         '<(skia_include_path)/core/SkThread.h',
         '<(skia_include_path)/core/SkTime.h',
         '<(skia_include_path)/core/SkTLazy.h',
         '<(skia_include_path)/core/SkTypeface.h',
         '<(skia_include_path)/core/SkTypes.h',
         '<(skia_include_path)/core/SkUnPreMultiply.h',
+        '<(skia_include_path)/core/SkUnitMapper.h',
         '<(skia_include_path)/core/SkUtils.h',
         '<(skia_include_path)/core/SkWeakRefCnt.h',
         '<(skia_include_path)/core/SkWriter32.h',
         '<(skia_include_path)/core/SkXfermode.h',
 
         # Lazy decoding:
         '<(skia_src_path)/lazy/SkCachingPixelRef.cpp',
         '<(skia_src_path)/lazy/SkCachingPixelRef.h',
 
         # Path ops
@@ skipping 46 matching lines @@
         '<(skia_src_path)/pathops/SkPathOpsPoint.h',
         '<(skia_src_path)/pathops/SkPathOpsQuad.h',
         '<(skia_src_path)/pathops/SkPathOpsRect.h',
         '<(skia_src_path)/pathops/SkPathOpsTriangle.h',
         '<(skia_src_path)/pathops/SkPathOpsTypes.h',
         '<(skia_src_path)/pathops/SkPathWriter.h',
         '<(skia_src_path)/pathops/SkQuarticRoot.h',
         '<(skia_src_path)/pathops/SkReduceOrder.h',
     ],
 }