Try to reland v8_snapshot GN build changes, take #3

snapshot_toolchain.gni

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-# The snapshot needs to be compiled for the host, but compiled with
-# a toolchain that matches the bit-width of the target.
+declare_args() {
+  # The snapshot needs to be compiled for the host, but compiled with
+  # a toolchain that matches the bit-width of the target.
+  v8_snapshot_toolchain = ""
+}
 
 # TODO(GYP): For now we only support 32-bit little-endian target builds from an
 # x64 Linux host. Eventually we need to support all of the host/target
 # configurations v8 runs on.
-if (host_cpu == "x64" && host_os == "linux") {
-  if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
-    snapshot_toolchain = "//build/toolchain/linux:clang_x86"
-  } else if (target_cpu == "x64" || target_cpu == "arm64" || target_cpu == "mips64el") {
-    snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+if (v8_snapshot_toolchain == "") {
+  if (host_cpu == "x64" && host_os == "linux") {
+    if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
+      if (target_os == "android" || is_clang) {
+        v8_snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+      } else if (target_os == "chromeos") {
+        # TODO(dpranke): crbug.com/608596: Remove this clause once the
+        # CrOS ebuilds are setting v8_snapshot_toolchain directly and
+        # we've cleaned up the sysroot settings in //build.
+        v8_snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+      } else {
+        v8_snapshot_toolchain = "//build/toolchain/linux:x86"
+      }
+
+    } else if (target_cpu == "x64" || target_cpu == "arm64" ||
+               target_cpu == "mips64el") {
+      if (target_os == "android" || is_clang) {
+        v8_snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+      } else if (target_os == "chromeos") {
+        # TODO(dpranke): crbug.com/608596: Remove this clause once the
+        # CrOS ebuilds are setting v8_snapshot_toolchain directly and
+        # we've cleaned up the sysroot settings in //build.
+        v8_snapshot_toolchain = "//build/toolchain/linux:clang_x64"
+      } else {
+        v8_snapshot_toolchain = "//build/toolchain/linux:x64"
+      }
+    } else {
+      assert(false, "Need environment for this arch: $target_cpu")
+    }
+  } else if (host_os == "mac" && target_os == "win") {
+    v8_snapshot_toolchain = "//build/toolchain/mac:clang_$target_cpu"
   } else {
-    assert(false, "Need environment for this arch: $target_cpu")
+    v8_snapshot_toolchain = default_toolchain
   }
-} else {
-  snapshot_toolchain = default_toolchain
 }
 
-
+# TODO(dpranke): snapshot_toolchain is provided for backwards compatibility
+# and should be removed once all callers are updated to refer to
+# v8_snapshot_toolchain directly.
+snapshot_toolchain = v8_snapshot_toolchain