Revert "Remove SkPDFDocument and SkPDFDevice from the public headers."

gyp/pdf.gyp

 # This file builds the PDF backend.
 {
   'targets': [
     {
       'target_name': 'pdf',
       'product_name': 'skia_pdf',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'skia_lib.gyp:skia_lib',
         'skflate.gyp:skflate',
       ],
       'includes': [
         'pdf.gypi',
       ],
       'include_dirs': [
+        '../include/pdf',
         '../src/core', # needed to get SkGlyphCache.h and SkTextFormatParams.h
         '../src/pdf',
         '../src/utils', # needed to get SkBitSet.h
       ],
       'sources': [
         'pdf.gypi', # Makes the gypi appear in IDEs (but does not modify the build).
       ],
       'conditions': [
         [ 'skia_android_framework', {
             # Add SFTNLY support for PDF (which in turns depends on ICU)
             'include_dirs': [
               'external/sfntly/cpp/src',
             ],
             'libraries': [
               'libsfntly.a',
               '-licuuc',
               '-licui18n',
             ],
           }
         ],
       ],
+      # This section makes all targets that depend on this target
+      # #define SK_SUPPORT_PDF and have access to the pdf header files.
       'direct_dependent_settings': {
         'defines': [
           'SK_SUPPORT_PDF',
         ],
+        'include_dirs': [
+          '../include/pdf',
+        ],
       },
     },
   ],
 }