Merged r15868, r15871, r15880, r15884 into trunk branch.

Setting the thread name may fail, so don't assert that the result is 0.

Removed pointer space to pointer space compaction check when migrating objects.

Ignore external strings in old pointer space migration invariant check.

Fix JSArray-specific length lookup in polymorphic array handling

BUG=chromium:263276
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/20329002

git-svn-id: https://v8.googlecode.com/svn/trunk@15886 ce2b1a6d-e550-0410-aec6-3dcde31c8c00