[proxies] Improve JSProxyVerify and test most proxy invariants.

BUG=

Review URL: https://codereview.chromium.org/1549793003

Cr-Commit-Position: refs/heads/master@{#33032}

src/objects-debug.cc

@@ -818,8 +818,14 @@ void JSRegExp::JSRegExpVerify() {
 
 void JSProxy::JSProxyVerify() {
   CHECK(IsJSProxy());
+  VerifyPointer(target());
   VerifyPointer(handler());
+  CHECK_EQ(target()->IsCallable(), map()->is_callable());
+  CHECK_EQ(target()->IsConstructor(), map()->is_constructor());
   CHECK(hash()->IsSmi() || hash()->IsUndefined());
+  CHECK(map()->prototype()->IsNull());
+  // There should be no properties on a Proxy.
+  CHECK_EQ(0, map()->NumberOfOwnDescriptors());
 }