Add option to disable MirrorCache.

test/mjsunit/debug-toggle-mirror-cache.js

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-// Flags: --allow-natives-syntax
+// Flags: --expose-debug-as debug
 
-var x = 0;
-var y = 0;
+var handle1 = debug.MakeMirror(123).handle();
+assertEquals("number", debug.LookupMirror(handle1).type());
 
-var thenable = { then: function(f) { x++; f(); } };
+debug.ToggleMirrorCache(false);
+var handle2 = debug.MakeMirror(123).handle();
+assertEquals(undefined, handle2);
+assertThrows(function() { debug.LookupMirror(handle2) });
 
-for (var i = 0; i < 3; ++i) {
-  Promise.resolve(thenable).then(function() { x++; y++; });
-}
-assertEquals(0, x);
-
-(function check() {
-  Promise.resolve().chain(function() {
-    // Delay check until all handlers have run.
-    if (y < 3) check(); else assertEquals(6, x);
-  }).catch(function(e) { %AbortJS("FAILURE: " + e) });
-})();
+debug.ToggleMirrorCache(true);
+var handle3 = debug.MakeMirror(123).handle();
+assertEquals("number", debug.LookupMirror(handle3).type());