Change GN defaults for toolchains for CrOS.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 92d0b6e12877497f30aafe083d2a08da38e4b4e7..a1042d110e7b70d27513ef71ada5712989e31260 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -156,11 +156,9 @@ if (host_toolchain == "") {
   # 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"