Ensure that JSProxy::Fix gives the generated JSObject map a constructor

All JSObjects in V8 either have a map()->constructor() field or are
JSFunctions. JSProxy::Fix, however, was not enforcing this, and
Object.observe's use of JSObject::GetCreationContext() exposed this.

Note that this is not Object.observe-specific: the API call
v8::Object::CreationContext() also would have revealed this bug.

This patch chooses Object as a reasonable constructor to put on the
newly-fixed object's map. Note that this has no effect on the "constructor"
property in JS. In doing so, I've also tightened up the code underlying
JSProxy::Fix to only support JSObject and JSFunction as possible output
types.

BUG=405844
LOG=N
R=rossberg@chromium.org, verwaest@chromium.org

Review URL: https://codereview.chromium.org/505303004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23466 ce2b1a6d-e550-0410-aec6-3dcde31c8c00