Value::IsObject should return true for proxies as well

Because in the C++ API, a Proxy is an Object

BUG=none
R=bmeurer@chromium.org
LOG=n
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng

Review URL: https://codereview.chromium.org/1590873003

Cr-Commit-Position: refs/heads/master@{#33329}

src/api.cc

@@ -2766,9 +2766,7 @@ bool Value::IsSharedArrayBuffer() const {
 }
 
 
-bool Value::IsObject() const {
-  return Utils::OpenHandle(this)->IsJSObject();
-}
+bool Value::IsObject() const { return Utils::OpenHandle(this)->IsJSReceiver(); }
 
 
 bool Value::IsNumber() const {