Fix a stray reference to 'build_os' in BUILD.gn.

This should be 'host_os'; the build_os variable is going away.

R=jochen@chromium.org
BUG=344767
LOG=n

Review URL: https://codereview.chromium.org/1010683003

Cr-Commit-Position: refs/heads/master@{#27302}

BUILD.gn

@@ -1396,7 +1396,7 @@ source_set("v8_libbase") {
   } else if (is_android) {
     defines += [ "CAN_USE_VFP_INSTRUCTIONS" ]
 
-    if (build_os == "mac") {
+    if (host_os == "mac") {
       if (current_toolchain == host_toolchain) {
         sources += [ "src/base/platform/platform-macos.cc" ]
       } else {