Fix third_party/closure_compiler/run_compiler script to run on Mac.

Bug: None
Change-Id: Ie24bfa53b494775cd6c3f140a6e5655086c37b6c
Reviewed-on: https://chromium-review.googlesource.com/570190Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486546}

third_party/closure_compiler/run_compiler

@@ -12,4 +12,11 @@ export GYP_GENERATORS="ninja"
 
 tools/gyp/gyp --no-circular-check third_party/closure_compiler/compiled_resources2.gyp
 
-ninja -C out/Default -j `nproc` $@
+os_name=`uname -s`
+cores_count=1
+case "$os_name" in
+  "Darwin") cores_count=`sysctl -n hw.ncpu` ;;
+  "Linux") cores_count=`nproc` ;;
+esac
+
+ninja -C out/Default -j $cores_count $@