Mjsunit/debug-script test fails intermittendly with --vector-stores

The problem is that the gc pattern is different, and the list of debug scripts
may have more or less duplicates than before. The solution is to just turn off
--stress-opt for the test.

R=yangguo@chromium.org
BUG=v8:4502
LOG=N

Review URL: https://codereview.chromium.org/1416883002

Cr-Commit-Position: refs/heads/master@{#31415}

test/mjsunit/debug-script.js

@@ -28,6 +28,11 @@
 // Flags: --expose-debug-as debug --expose-gc --send-idle-notification
 // Flags: --allow-natives-syntax --expose-natives-as natives
 // Flags: --noharmony-shipping
+// Flags: --nostress-opt
+
+// --nostress-opt is specified because in stress mode the compilation cache
+// may hold on to old copies of scripts (see bug 1641).
+
 // Note: this test checks that that the number of scripts reported as native
 // by Debug.scripts() is the same as a number of core native scripts.
 // Native scripts that are added by --harmony-shipping are classified