Land prep work to enable NaCl in the Linux x64 GN builds.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index a554f12dc5b427ddbff8a0fff1116eb9bc6cbbdd..7c06e5934c8c53a569df09179c8bfa5f0cd62ac5 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -548,6 +548,12 @@ if (is_win) {
 } else if (is_ios) {
   host_toolchain = "//build/toolchain/mac:host_clang"
   set_default_toolchain("//build/toolchain/mac:clang")
+} else if (is_nacl) {
+  # TODO(GYP): This will need to change when we get NaCl working
+  # on multiple platforms, but this whole block of code (how we define
+  # host_toolchain) needs to be reworked regardless to key off of build_os
+  # and build_cpu_arch rather than the is_* variables.
+  host_toolchain = "//build/toolchain/linux:clang_x64"
 }
 
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