SkPDF: allow PDF module to be disabled in DM and SampleApp

gyp/pdf.gyp

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 # This file builds the PDF backend.
 {
   'targets': [
     {
+      'target_name': 'nopdf',
+      'type': 'static_library',
+      'dependencies': [ 'skia_lib.gyp:skia_lib', ],
+      'sources': [ '<(skia_src_path)/doc/SkDocument_PDF_None.cpp', ],
+      'defines': [ 'SK_SUPPORT_PDF=0', ],
+    },
+    {
       'target_name': 'pdf',
       'product_name': 'skia_pdf',
       'type': 'static_library',
       'standalone_static_library': 1,
       'variables': { 'skia_pdf_use_sfntly%': 1, },
       'dependencies': [
         'skia_lib.gyp:skia_lib',
         'skflate.gyp:skflate',
       ],
       'includes': [
@@ skipping 20 matching lines @@
             ],
             'libraries': [
               'libsfntly.a',
               '-licuuc',
               '-licui18n',
             ],
           }
         ],
       ],
       'direct_dependent_settings': {
+        'defines': [ 'SK_SUPPORT_PDF=1', ],
         'include_dirs': [
           '../include/core',  # SkDocument.h
         ],
       },
     },
   ],
 }