Prepare for -Wall for third-party code, -Wextra for chromium_code

third_party/mach_override/mach_override.gyp

Index: third_party/mach_override/mach_override.gyp
diff --git a/third_party/mach_override/mach_override.gyp b/third_party/mach_override/mach_override.gyp
index b16df0b22824a0117885f91e0084a4a3512eb41d..7206a0a955fd666b33c2cc13cc21f5b5e815480f 100644
--- a/third_party/mach_override/mach_override.gyp
+++ b/third_party/mach_override/mach_override.gyp
@@ -47,6 +47,13 @@
               # For UD_ASSERT(!"message");
               '-Wstring-conversion',
             ],
+            'clang_warning_flags': [
+              # syn.c contains a switch with an assert(false) in a default:
+              # block.  In release builds, the function is missing a return.
+              '-Wno-return-type',
+              # Fires once in decode.c.
+              '-Wno-sometimes-uninitialized',
+            ],
           },
         },
         {