clang/win: Build chromium code without -Wno-incompatible-pointer-types.

third_party/mesa/BUILD.gn

Index: third_party/mesa/BUILD.gn
diff --git a/third_party/mesa/BUILD.gn b/third_party/mesa/BUILD.gn
index 2d4ec15e9be3fccb2e16559c429f9891e94fd6be..67caf8ecaca3270271d501a80d21ee59a2ec3f6c 100644
--- a/third_party/mesa/BUILD.gn
+++ b/third_party/mesa/BUILD.gn
@@ -644,8 +644,15 @@ static_library("mesa") {
   }
 
   if (is_win) {
-    # Because we're building as a static library
-    defines = [ "_GLAPI_NO_EXPORTS" ]
+    defines = [
+      # Because we're building as a static library
+      "_GLAPI_NO_EXPORTS",
+
+      # Generated files use const only if __cplusplus or __STDC__ is defined.
+      # On Windows, neither is defined, so define YY_USE_CONST to explicitly
+      # enable const.
+      "YY_USE_CONST",
+    ]
   }
 
   deps = [