Change " to ' in gyp file for consistency

pdfium.gyp

Index: pdfium.gyp
diff --git a/pdfium.gyp b/pdfium.gyp
index 1fc54ea2eb7c9350a3ec96fb0c0d9700c287ad00..87c4fc6b4a238455927fd98228d8468bed6f4c06 100644
--- a/pdfium.gyp
+++ b/pdfium.gyp
@@ -1121,25 +1121,25 @@
             '<(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',
               ]
             }],
           ],