WebPageProxy::close() is a no-op for terminated processes

WebPageProxy::close() is a no-op for terminated processes
https://bugs.webkit.org/show_bug.cgi?id=136378
Related to <rdar://problem/16991213> and to <rdar://problem/17095600>

Reviewed by Brady Eidson.

Also fixes issues that got uncovered after making close() work.

* UIProcess/WebInspectorProxy.cpp: (WebKit::WebInspectorProxy::invalidate): Don't
close the page, because it makes no sense, and causes an assertion now. Previosly,
this was OK because the page was invalid already, and close() was a no-op.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::~WebPageProxy): Added some assertions to catch invalidation
issues earlier.
(WebKit::WebPageProxy::reattachToWebProcess): Make it an invariant that a page's
process always has a message receiver for it, until close() removes it.
(WebKit::WebPageProxy::close): Make this function work for all open pages, whether
they have a page or not.
(WebKit::WebPageProxy::processDidFinishLaunching): Added an asserion that process
agrees about its state.
(WebKit::WebPageProxy::resetStateAfterProcessExited): Don't remove a message receiver,
we now only do this in reattach or close.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@173118 268f45cc-cd09-0410-ab3c-d52691b4dbfc