GN: Propagate top-level host_toolchain through to all toolchains

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 3eed120832f10c091058706af881c439adc0454c..16e49b87a7ef76842fdd0fd8604f4309018b4d69 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -129,6 +129,10 @@ declare_args() {
   # argument, and set as the default toolchain.
   custom_toolchain = ""
 
+  # This should not normally be set as a build argument.  It's here so that
+  # every toolchain can pass through the "global" value via toolchain_args().
+  host_toolchain = ""
+
   # DON'T ADD MORE FLAGS HERE. Read the comment above.
 }
 
@@ -144,30 +148,38 @@ declare_args() {
 # We do this before anything else to make sure we complain about any
 # unsupported os/cpu combinations as early as possible.
 
-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?
-    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
-  # use the target_cpu instead of the host_cpu.
-  if (is_clang) {
-    host_toolchain = "//build/toolchain/win:clang_$target_cpu"
+if (host_toolchain == "") {
+  # This should only happen in the top-level context.
+  # In a specific toolchain context, the toolchain_args()
+  # block should have propagated a value down.
+  # TODO(dpranke): Add some sort of assert here that verifies that
+  # no toolchain omitted host_toolchain from its toolchain_args().
+
+  if (host_os == "linux") {
+    if (target_os != "linux") {
+      # TODO(dpranke) - is_clang normally applies only to the target
+      # build, and there is no way to indicate that you want to override
+      # it for both the target build *and* the host build. Do we need to
+      # support this?
+      host_toolchain = "//build/toolchain/linux:clang_$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_$host_cpu"
+  } else if (host_os == "win") {
+    # TODO(crbug.com/467159): win cross-compiles don't actually work yet, so
+    # use the target_cpu instead of the host_cpu.
+    if (is_clang) {
+      host_toolchain = "//build/toolchain/win:clang_$target_cpu"
+    } else {
+      host_toolchain = "//build/toolchain/win:$target_cpu"
+    }
   } else {
-    host_toolchain = "//build/toolchain/win:$target_cpu"
+    assert(false, "Unsupported host_os: $host_os")
   }
-} else {
-  assert(false, "Unsupported host_os: $host_os")
 }
 
 _default_toolchain = ""
@@ -200,7 +212,7 @@ if (target_os == "android") {
   assert(host_os == "mac", "Mac cross-compiles are unsupported.")
   _default_toolchain = host_toolchain
 } else if (target_os == "win") {
-  # On windows we use the same toolchain for host and target by default.
+  # On Windows we use the same toolchain for host and target by default.
   assert(target_os == host_os, "Win cross-compiles only work on win hosts.")
   if (is_clang) {
     _default_toolchain = "//build/toolchain/win:clang_$target_cpu"