Change GN defaults for toolchains for CrOS.

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 138 matching lines @@
 # unsupported os/cpu combinations as early as possible.
 
 if (host_toolchain == "") {
   # This should only happen in the top-level context.
   # In a specific toolchain context, the toolchain_args()
   # block should have propagated a value down.
   # TODO(dpranke): Add some sort of assert here that verifies that
   # no toolchain omitted host_toolchain from its toolchain_args().
 
   if (host_os == "linux") {
-    if (target_os != "linux") {
-      # TODO(dpranke) - is_clang normally applies only to the target
-      # build, and there is no way to indicate that you want to override
-      # it for both the target build *and* the host build. Do we need to
-      # support this?
+    if (target_os == "android") {
+      # We want android builds to use clang for the host toolchains,
+      # but for CrOS we want is_clang to apply to both the target and host.
       host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
     } else if (is_clang) {
       host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
     } else {
       host_toolchain = "//build/toolchain/linux:$host_cpu"
     }
   } else if (host_os == "mac") {
     host_toolchain = "//build/toolchain/mac:clang_$host_cpu"
   } else if (host_os == "win") {
     # TODO(crbug.com/467159): win cross-compiles don't actually work yet, so
@@ skipping 446 matching lines @@
     forward_variables_from(invoker, "*", [ "visibility" ])
 
     # All shared libraries must have the sanitizer deps to properly link in
     # asan mode (this target will be empty in other cases).
     if (!defined(deps)) {
       deps = []
     }
     deps += [ "//build/config/sanitizers:deps" ]
   }
 }