Enable cross compile to android from mac.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 2274c0e98c3266160a8acce3491f5fe269ec45be..ee32088dfcdee63f409cff05760d4a8261d92680 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -488,11 +488,17 @@ if (is_win) {
   }
   set_default_toolchain("$host_toolchain")
 } else if (is_android) {
-  # Use clang for the x86/64 Linux host builds.
-  if (host_cpu == "x86" || host_cpu == "x64") {
-    host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
+  if (host_os == "linux") {
+    # Use clang for the x86/64 Linux host builds.
+    if (host_cpu == "x86" || host_cpu == "x64") {
+      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_$host_cpu"
   } else {
-    host_toolchain = "//build/toolchain/linux:$host_cpu"
+    assert(false, "Unknown host for android cross compile")
   }
   set_default_toolchain("//build/toolchain/android:$current_cpu")
 } else if (is_linux) {
@@ -507,11 +513,11 @@ if (is_win) {
     set_default_toolchain("//build/toolchain/cros:target")
   }
 } else if (is_mac) {
-  host_toolchain = "//build/toolchain/mac:clang"
+  host_toolchain = "//build/toolchain/mac:clang_x64"
   set_default_toolchain(host_toolchain)
 } else if (is_ios) {
-  host_toolchain = "//build/toolchain/mac:host_clang"
-  set_default_toolchain("//build/toolchain/mac:clang")
+  host_toolchain = "//build/toolchain/mac:clang_x64"
+  set_default_toolchain("//build/toolchain/mac:clang_$current_cpu")
 } 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