Implements the strong mode proposal's restrictions on changing a strong object's
writable, non-configurable property to non-writable.
Setting the strong bit is still wip, so this change will only affect those
objects that have the bit correctly set. The tests reflect this, and will be
expanded as more objects can be marked as strong.
BUG=v8:3956
LOG=N
Review URL: https://codereview.chromium.org/1142393003
Cr-Commit-Position: refs/heads/master@{#28698}
