Use newSingleThreadExecutor instead of the unbounded newCachedThreadP…

…ool so that the thread count is limited to one

We can't shutdown the executor easily, because people reuse the compiler instance. Note that the executor shouldn't be accepting more than one task at a time anyway so this change should be safe.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160703835

src/com/google/javascript/jscomp/CompilerExecutor.java

@@ -66,7 +66,7 @@ final class CompilerExecutor {
    */
   @GwtIncompatible("java.util.concurrent.ExecutorService")
   static ExecutorService getDefaultExecutorService() {
-    return Executors.newCachedThreadPool(new ThreadFactory() {
+    return Executors.newSingleThreadExecutor(new ThreadFactory() {
         @Override
         public Thread newThread(Runnable r) {
           Thread t = new Thread(null, r, "jscompiler", COMPILER_STACK_SIZE);