Have JSProxy and JSGlobalProxy use the properties or hash technology
like we use for all other JSReceivers. Also unify and simplify the
code dealing with these hashes.

Bug: v8:6344, v8:6911
Change-Id: Ic995639c74211ba6f33acd73428b8c6d95bf7919
Reviewed-on: https://chromium-review.googlesource.com/737833Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48930}