Windows GN clobber error fixes

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index beee0781d673a1ae2e453d07df65371339009c99..40420c110c7125fb951c13be6d3ad6e5c541e915 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -238,11 +238,12 @@ template("msvc_toolchain") {
         current_os = invoker.toolchain_os
       }
 
-      forward_variables_from(invoker,
-                             [
-                               "is_clang",
-                               "is_component_build",
-                             ])
+      if (defined(invoker.is_clang)) {
+        is_clang = invoker.is_clang
+      }
+      if (defined(invoker.is_component_build)) {
+        is_component_build = invoker.is_component_build
+      }

[Dirk Pranke, 2016/05/03 17:48:50]

I'm confused; isn't this exactly the same?

[brettw, 2016/05/03 18:00:43]

With the new changes to forward_variables_from, it won't clobber a variable
implicitly. In the rare cases where you want to clobber, you need to do it
explicitly which is what happens here.

[Dirk Pranke, 2016/05/03 20:06:33]

Ah, right, I was thinking they weren't already in scope but I wasn't thinking of
the top level declared_args.

Maybe add a comment indicating why using forward_variables_from() is the wrong
thing to do here and below, since that's non-obvious (although probably true for
all toolchain() rules)?

 
       # This value needs to be passed through unchanged.
       host_toolchain = host_toolchain
@@ -313,7 +314,9 @@ template("win_x64_toolchains") {
     cl = "${goma_prefix}\"${x64_toolchain_data.vc_bin_dir}/cl.exe\""
     is_clang = false
 
-    forward_variables_from(invoker, [ "is_component_build" ])
+    if (defined(invoker.is_component_build)) {
+      is_component_build = invoker.is_component_build
+    }
   }
 
   msvc_toolchain("clang_" + target_name) {
@@ -325,7 +328,9 @@ template("win_x64_toolchains") {
     toolchain_os = "win"
     is_clang = true
 
-    forward_variables_from(invoker, [ "is_component_build" ])
+    if (defined(invoker.is_component_build)) {
+      is_component_build = invoker.is_component_build
+    }
   }
 }