Resolve all but two VC++ build warnings in pdfium.

build/standalone.gypi

Index: build/standalone.gypi
diff --git a/build/standalone.gypi b/build/standalone.gypi
index d335f05e49c5ce1b6c597f387eb8c60764b889e7..5271350e831125ed60e98c68c835659f617572eb 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -165,6 +165,7 @@
       'IntermediateDirectory': '$(OutDir)\\obj\\$(ProjectName)',
       'CharacterSet': '1',
     },
+    'msvs_disabled_warnings': [4800],
     'msvs_settings': {
       'VCCLCompilerTool': {
         'MinimalRebuild': 'false',
@@ -172,7 +173,6 @@
         'EnableFunctionLevelLinking': 'true',
         'RuntimeTypeInfo': 'false',
         'WarningLevel': '3',
-        'WarnAsError': 'false',
         'DebugInformationFormat': '3',
         'Detect64BitPortabilityProblems': 'false',
         'conditions': [
@@ -184,6 +184,12 @@
           }, {
             'ExceptionHandling': '0',
           }],
+          ['target_arch=="x64"', {
+            'WarnAsError': 'false',
+          }, {
+            'WarnAsError': 'true',
+          }
+          ]
         ],
       },
       'VCLibrarianTool': {