Change " to ' in gyp file for consistency

pdfium.gyp

 # Copyright 2015 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'chromium_code': 1,
     'pdf_use_skia%': 0,
     'pdf_enable_v8%': 1,
     'pdf_enable_xfa%': 0, # Set to 1 by standalone.gypi in a standalone build.
@@ skipping 1103 matching lines @@
             '<(DEPTH)/v8',
             '<(DEPTH)/v8/include',
           ],
           'dependencies': [
             '<(DEPTH)/v8/src/v8.gyp:v8',
           ],
           'export_dependent_settings': [
             '<(DEPTH)/v8/src/v8.gyp:v8',
           ],
           "sources": [
-            "fxjs/fxjs_v8.cpp",
-            "fxjs/include/fxjs_v8.h",
+            'fxjs/fxjs_v8.cpp',
+            'fxjs/include/fxjs_v8.h',
           ],
           "conditions": [
             ['pdf_enable_xfa==1', {
               'sources': [
-                "fxjs/cfxjse_arguments.cpp",
-                "fxjs/cfxjse_class.cpp",
-                "fxjs/cfxjse_context.cpp",
-                "fxjs/cfxjse_isolatetracker.cpp",
-                "fxjs/cfxjse_isolatetracker.h",
-                "fxjs/cfxjse_runtimedata.cpp",
-                "fxjs/cfxjse_runtimedata.h",
-                "fxjs/cfxjse_value.cpp",
-                "fxjs/include/cfxjse_arguments.h",
-                "fxjs/include/cfxjse_class.h",
-                "fxjs/include/cfxjse_context.h",
-                "fxjs/include/cfxjse_value.h",
-                "fxjs/include/fxjse.h",
+                'fxjs/cfxjse_arguments.cpp',
+                'fxjs/cfxjse_class.cpp',
+                'fxjs/cfxjse_context.cpp',
+                'fxjs/cfxjse_isolatetracker.cpp',
+                'fxjs/cfxjse_isolatetracker.h',
+                'fxjs/cfxjse_runtimedata.cpp',
+                'fxjs/cfxjse_runtimedata.h',
+                'fxjs/cfxjse_value.cpp',
+                'fxjs/include/cfxjse_arguments.h',
+                'fxjs/include/cfxjse_class.h',
+                'fxjs/include/cfxjse_context.h',
+                'fxjs/include/cfxjse_value.h',
+                'fxjs/include/fxjse.h',
               ]
             }],
           ],
         },
       ]
     }],
     ['pdf_enable_xfa==1', {
       'targets': [
         {
           'target_name': 'fpdfxfa',
@@ skipping 11 matching lines @@
             'fpdfsdk/fpdfxfa/include/fpdfxfa_app.h',
             'fpdfsdk/fpdfxfa/include/fpdfxfa_doc.h',
             'fpdfsdk/fpdfxfa/include/fpdfxfa_page.h',
             'fpdfsdk/fpdfxfa/include/fpdfxfa_util.h',
           ],
         },
       ]
     }],
   ]
 }