Turn on warnings as errors for (most) third_party code on Windows

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 7fd9f21b3a6f2d9c06ebb52a66cdf6edb3a6db0b..4de35f9af7d4d533e492508abe8b5776ce762ef1 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -707,6 +707,14 @@
     # to get incremental linking to be faster in debug builds.
     'incremental_chrome_dll%': '0',
 
+    # The default settings for third party code for treating
+    # warnings-as-errors. Ideally, this would not be required, however there
+    # is some third party code that takes a long time to fix/roll. So, this
+    # flag allows us to have warnings as errors in general to prevent
+    # regressions in most modules, while working on the bits that are
+    # remaining.
+    'win_third_party_warn_as_error%': 'true',
+
     # This is the location of the sandbox binary. Chrome looks for this before
     # running the zygote process. If found, and SUID, it will be used to
     # sandbox the zygote process and, thus, all renderer processes.
@@ -1660,7 +1668,7 @@
             'msvs_settings': {
               'VCCLCompilerTool': {
                 'WarningLevel': '3',
-                'WarnAsError': 'false', # TODO(maruel): Enable it.
+                'WarnAsError': '<(win_third_party_warn_as_error)',
                 'Detect64BitPortabilityProblems': 'false',
               },
             },