Increase warning level to 4 on Visual Studio

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 38fc7230a45c8ea765892cffc6c5d52c2d15a493..4086ec7f063c81daee9cfa397659edff885cdcb3 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -758,7 +758,8 @@
             'msvs_disabled_warnings': [4800],
             'msvs_settings': {
               'VCCLCompilerTool': {
-                'WarnAsError': 'false',
+                'WarningLevel': '3',
+                'WarnAsError': 'false', # TODO(maruel): Enable it.
                 'Detect64BitPortabilityProblems': 'false',
               },
             },
@@ -1520,14 +1521,20 @@
           '$(VSInstallDir)/VC/atlmfc/include',
         ],
         'msvs_cygwin_dirs': ['<(DEPTH)/third_party/cygwin'],
-        'msvs_disabled_warnings': [4351, 4396, 4503, 4819],
+        'msvs_disabled_warnings': [4351, 4396, 4503, 4819,
+          # TODO(maruel): These warnings are level 4. They will be slowly
+          # removed as code is fixed.
+          4100, 4121, 4125, 4127, 4130, 4131, 4189, 4201, 4238, 4244, 4245,
+          4310, 4355, 4428, 4481, 4505, 4510, 4512, 4530, 4610, 4611, 4701,
+          4702, 4706,
+        ],
         'msvs_settings': {
           'VCCLCompilerTool': {
             'MinimalRebuild': 'false',
             'BufferSecurityCheck': 'true',
             'EnableFunctionLevelLinking': 'true',
             'RuntimeTypeInfo': 'false',
-            'WarningLevel': '3',
+            'WarningLevel': '4',
             'WarnAsError': 'true',
             'DebugInformationFormat': '3',
             'conditions': [