Roll pdfium, add GN build for pdf and pdfium.

pdf/pdf.gyp

 {
   'variables': {
     'chromium_code': 1,
     'pdf_engine%': 0,  # 0 PDFium
   },
   'target_defaults': {
     'cflags': [
       '-fPIC',
     ],
   },
   'targets': [
     {
       'target_name': 'pdf',
       'type': 'loadable_module',
       'msvs_guid': '647863C0-C7A3-469A-B1ED-AD7283C34BED',
       'dependencies': [
         '../base/base.gyp:base',
         '../net/net.gyp:net',
         '../ppapi/ppapi.gyp:ppapi_cpp',
-        '../third_party/pdfium/pdfium.gyp:fpdfsdk',
+        '../third_party/pdfium/pdfium.gyp:pdfium',
       ],
       'xcode_settings': {
         'INFOPLIST_FILE': 'Info.plist',
       },
       'mac_framework_dirs': [
         '$(SDKROOT)/System/Library/Frameworks/ApplicationServices.framework/Frameworks',
       ],
       'ldflags': [ '-L<(PRODUCT_DIR)',],
       'sources': [
         'button.h',
@@ skipping 129 matching lines @@
                 'pdf',
                 '../breakpad/breakpad.gyp:dump_syms',
               ],
             }],
           ],
         },
       ],
     },],  # OS=="linux" and chromeos==0
   ],
 }