Author: Olivier Tilloy <olivier.tilloy@canonical.com>
Description: Do not use warning flags that require a newer Clang (bionic has 6.0.0,
  see https://releases.llvm.org/6.0.0/tools/clang/docs/DiagnosticsReference.html)

--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -668,8 +668,7 @@ config("compiler") {
   # This flag enforces that member pointer base types are complete. It helps
   # prevent us from running into problems in the Microsoft C++ ABI (see
   # https://crbug.com/847724).
-  if (is_clang && !is_nacl && target_os != "chromeos" && !use_xcode_clang &&
-      (is_win || use_custom_libcxx)) {
+  if (false) {
     cflags += [ "-fcomplete-member-pointers" ]
   }
 
@@ -1468,7 +1467,7 @@ config("default_warnings") {
 
           # Ignore warnings about MSVC optimization pragmas.
           # TODO(thakis): Only for no_chromium_code? http://crbug.com/505314
-          "-Wno-ignored-pragma-optimize",
+          #"-Wno-ignored-pragma-optimize",
         ]
       }
     }