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 5ac58b68ea019e758304d19373a61976fa846d4d..829f0fea7edec059a94ac346335df0cd9c8f3e87 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -710,6 +710,14 @@
     # 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.
@@ -1672,7 +1680,7 @@
             'msvs_settings': {
               'VCCLCompilerTool': {
                 'WarningLevel': '3',
-                'WarnAsError': 'false', # TODO(maruel): Enable it.
+                'WarnAsError': '<(win_third_party_warn_as_error)',
                 'Detect64BitPortabilityProblems': 'false',
               },
             },