Move function prototypes to new doc_utils.h file

pdfium.gyp

 {
   'variables': {
     'pdf_use_skia%': 0,
     'conditions': [
       ['OS=="linux"', {
         'bundle_freetype%': 0,
       }, {  # On Android there's no system FreeType. On Windows and Mac, only a
             # few methods are used from it.
         'bundle_freetype%': 1,
       }],    
@@ skipping 157 matching lines @@
         'core/src/fpdfdoc/doc_basic.cpp',
         'core/src/fpdfdoc/doc_bookmark.cpp',
         'core/src/fpdfdoc/doc_form.cpp',
         'core/src/fpdfdoc/doc_formcontrol.cpp',
         'core/src/fpdfdoc/doc_formfield.cpp',
         'core/src/fpdfdoc/doc_link.cpp',
         'core/src/fpdfdoc/doc_metadata.cpp',
         'core/src/fpdfdoc/doc_ocg.cpp',
         'core/src/fpdfdoc/doc_tagged.cpp',
         'core/src/fpdfdoc/doc_utils.cpp',
+        'core/src/fpdfdoc/doc_utils.h',
         'core/src/fpdfdoc/doc_viewerPreferences.cpp',
         'core/src/fpdfdoc/doc_vt.cpp',
         'core/src/fpdfdoc/doc_vtmodule.cpp',
         'core/src/fpdfdoc/pdf_vt.h',
         'core/src/fpdfdoc/tagged_int.h',
       ],
     },
     {
       'target_name': 'fpdfapi',
       'type': 'static_library',
@@ skipping 675 matching lines @@
         'fpdfsdk/src/fpdfview_embeddertest.cpp',
         'testing/embedder_test.cpp',
         'testing/embedder_test.h',
         'testing/embedder_test_mock_delegate.h',
         'testing/fx_string_testhelpers.cpp',
         'testing/fx_string_testhelpers.h',
       ],
     },
   ],
 }