Do not modify FLAG_parallel_recompilation after start up.

test/mjsunit/assert-opt-and-deopt.js

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 // modification, are permitted provided that the following conditions are
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-// Flags: --allow-natives-syntax
-%SetFlags("--noparallel-recompilation");
+// Flags: --allow-natives-syntax --noparallel-recompilation
 
 /**
  * This class shows how to use %GetOptimizationCount() and
  * %GetOptimizationStatus() to infer information about opts and deopts.
  * Might be nice to put this into mjsunit.js, but that doesn't depend on
  * the --allow-natives-syntax flag so far.
  */
 function OptTracker() {
   this.opt_counts_ = {};
 }
@@ skipping 129 matching lines @@
 // Let's trigger optimization for another type.
 for (var i = 0; i < 5; i++) f("a");
 
 %OptimizeFunctionOnNextCall(f);
 f("b");
 
 tracker.AssertOptCount(f, 2);
 tracker.AssertIsOptimized(f, true);
 tracker.AssertDeoptHappened(f, true);
 tracker.AssertDeoptCount(f, 1);