Removing and re-adding a script message handler with the same name results in an...

Removing and re-adding a script message handler with the same name results in an unusable message handler
https://bugs.webkit.org/show_bug.cgi?id=155223

Reviewed by Sam Weinig.
Source/WebCore:

New API test: WKUserContentController.ScriptMessageHandlerReplaceWithSameName.

* page/UserMessageHandler.h:
(WebCore::UserMessageHandler::descriptor):
* page/UserMessageHandlersNamespace.cpp:
(WebCore::UserMessageHandlersNamespace::handler):
This lazy removal mechanism combined with the fact that we only compare
handler name and world makes it such that m_messageHandlers could have
a stale UserMessageHandler with a UserMessageHandlerDescriptor that differed
only in client.

It is safe to compare the descriptors by pointer instead because m_messageHandler
holds a strong reference to its UserMessageHandlerDescriptors, and this will ensure
that the add-remove-add path (with identical name and world) causes a new
UserContentController to be created.

We also now clean up any stale UserMessageHandlers whenever we're about to
add a new one, by removing any which the UserContentController no longer knows about.

Tools:

* TestWebKitAPI/Tests/WebKit2Cocoa/UserContentController.mm:
(TEST):
Add a test ensuring that it is possible to remove and re-add a script message handler
with the same name and still dispatch messages to it.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@197868 268f45cc-cd09-0410-ab3c-d52691b4dbfc