Before this, only the [[ProxyHandler]] was set to null during revocation
of the Proxy through either the v8::Proxy::Revoke() or the
Proxy.revocable() API. To be consistent with the spec, the Proxy's
target is set to null as well. This change should not be observable
through JS, since the check for if the Proxy is revoked should always
use the handler. But the changed value is exposed through the public
v8::Proxy::GetTarget() API, which is used by the inspector API and
Node.js.

Also included is a much more comprehensive test for Inspector's support
for Proxy, which prior to this commit did not work as intended.

Bug: 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I727607ec2b3cea8642cd636573932c1e6bb5cc07
Reviewed-on: https://chromium-review.googlesource.com/854676
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50476}