Fixes the implementation of step 9 in the Proxy's internal [[Get]]
method:

Let targetDesc be ? target.[[GetOwnProperty]](P)

If P is an accessor, this should not result in a call to the getter.
Likewise in [[Set]].

https://tc39.github.io/ecma262/#sec-proxy-object-internal-methods-and-internal-slots-get-p-receiver

Bug: chromium:776338
Change-Id: Ic06b7eeac6a1ef9606ddda6fa9d6d58b709702fb
Reviewed-on: https://chromium-review.googlesource.com/731123Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48813}