GN: forward_variables_from shouldn't clobber vars.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 2840ae0adf9f2d2298f43a6a26c9ee99d128f35d..4ef118b797cefabee46b8ea9075700e26135dbb5 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -412,14 +412,21 @@ template("gcc_toolchain") {
 
       forward_variables_from(invoker,
                              [
-                               "is_clang",
-                               "is_component_build",
-                               "is_nacl_glibc",
                                "use_allocator",
                                "use_gold",
                                "symbol_level",
                              ])
 
+      if (defined(invoker.is_clang)) {
+        is_clang = invoker.is_clang

[agrieve, 2016/05/03 00:14:09]

why did these need to be broken out?

[brettw, 2016/05/03 01:02:44]

These are all global variables and in this case we specifically want to clobber
if they invoker ones are defined.

+      }
+      if (defined(invoker.is_component_build)) {
+        is_component_build = invoker.is_component_build
+      }
+      if (defined(invoker.is_nacl_glibc)) {
+        is_nacl_glibc = invoker.is_nacl_glibc
+      }
+
       if (defined(invoker.clear_sanitizers) && invoker.clear_sanitizers) {
         is_asan = false
         is_cfi = false