Test mjsunit/allocation-site-info is flaky without explicit gc()

It started failing on arm64 nosnap during a test case where complex nested
literal arrays are created.

R=machenbach@chromium.org
BUG=

Review URL: https://codereview.chromium.org/367703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22125 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/mjsunit/allocation-site-info.js

@@ -414,6 +414,11 @@ if (support_smi_only_arrays) {
    assertKind(elements_kind.fast, obj[1][0]);
   })();
 
+  // Perform a gc because without it the test below can experience an
+  // allocation failure at an inconvenient point. Allocation mementos get
+  // cleared on gc, and they can't deliver elements kind feedback when that
+  // happens.
+  gc();
 
   // Make sure object literals with array fields benefit from the type feedback
   // that allocation mementos provide.