[test] Speed-up regression test for growing stores.

TBR=machenbach@chromium.org
BUG=chromium:635798,chromium:638295

Review-Url: https://codereview.chromium.org/2288813003
Cr-Commit-Position: refs/heads/master@{#38991}

test/mjsunit/regress/regress-crbug-635798.js

@@ -8,7 +8,7 @@ function foo() {
   var x = [];
   var y = [];
   x.__proto__ = y;
-  for (var i = 0; i < 200000; ++i) {
+  for (var i = 0; i < 10000; ++i) {
     y[i] = 1;
   }
 }