Add SkPDFResourceDict class, refactor existing code

gyp/pdf.gyp

 {
   'targets': [
     {
       'target_name': 'pdf',
       'product_name': 'skia_pdf',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'skia_lib.gyp:skia_lib',
         'zlib.gyp:zlib',
@@ skipping 20 matching lines @@
         '../src/pdf/SkPDFFormXObject.cpp',
         '../src/pdf/SkPDFFormXObject.h',
         '../src/pdf/SkPDFGraphicState.cpp',
         '../src/pdf/SkPDFGraphicState.h',
         '../src/pdf/SkPDFImage.cpp',
         '../src/pdf/SkPDFImage.h',
         '../src/pdf/SkPDFImageStream.cpp',
         '../src/pdf/SkPDFImageStream.h',
         '../src/pdf/SkPDFPage.cpp',
         '../src/pdf/SkPDFPage.h',
+        '../src/pdf/SkPDFResourceDict.cpp',
+        '../src/pdf/SkPDFResourceDict.h',
         '../src/pdf/SkPDFShader.cpp',
         '../src/pdf/SkPDFShader.h',
         '../src/pdf/SkPDFStream.cpp',
         '../src/pdf/SkPDFStream.h',
         '../src/pdf/SkPDFTypes.cpp',
         '../src/pdf/SkPDFTypes.h',
         '../src/pdf/SkPDFUtils.cpp',
         '../src/pdf/SkPDFUtils.h',
         '../src/pdf/SkTSet.h',
 
@@ skipping 11 matching lines @@
       },
     },
   ],
 }
 
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