Modern IDB: Cursors (still) do not keep their opening request alive.

Modern IDB: Cursors (still) do not keep their opening request alive.
https://bugs.webkit.org/show_bug.cgi?id=153724

Reviewed by Alex Christensen.

No new tests (All existing tests pass without flakiness).

IDBCursors did not properly keep their JS wrappers alive.
Making them ActiveDOMObjects that keep track of how many requests might be in flight fixes this.
This also makes them actually keep their opening-request live via the opaque-root mechanism.

IDBCursorWithValue also needed to opt in to all of these mechanisms.

* CMakeLists.txt:
* WebCore.xcodeproj/project.pbxproj:

* Modules/indexeddb/IDBCursor.h:
(WebCore::IDBCursor::hasPendingActivity): The base IDBCursor always has no pending activity,
  to maintain current behavior in LegacyIDB. This weirdness will go away when LegacyIDB does.
* Modules/indexeddb/IDBCursor.idl:
* Modules/indexeddb/IDBCursorWithValue.idl:

Track a count for all outstanding requests to keep the cursor alive as an ActiveDOMObject.
* Modules/indexeddb/client/IDBCursorImpl.cpp:
(WebCore::IDBClient::IDBCursor::IDBCursor):
(WebCore::IDBClient::IDBCursor::update):
(WebCore::IDBClient::IDBCursor::uncheckedIterateCursor):
(WebCore::IDBClient::IDBCursor::deleteFunction):
(WebCore::IDBClient::IDBCursor::activeDOMObjectName):
(WebCore::IDBClient::IDBCursor::canSuspendForDocumentSuspension):
(WebCore::IDBClient::IDBCursor::hasPendingActivity):
(WebCore::IDBClient::IDBCursor::decrementOutstandingRequestCount):
* Modules/indexeddb/client/IDBCursorImpl.h:

Rework the "delete" family of functions on the object store to allow for returning a modern IDBRequest.
A lot of this can go away when LegacyIDB does.
* Modules/indexeddb/client/IDBObjectStoreImpl.cpp:
(WebCore::IDBClient::IDBObjectStore::deleteFunction):
(WebCore::IDBClient::IDBObjectStore::doDelete):
(WebCore::IDBClient::IDBObjectStore::modernDelete):
* Modules/indexeddb/client/IDBObjectStoreImpl.h:

* Modules/indexeddb/client/IDBRequestImpl.cpp:
(WebCore::IDBClient::IDBRequest::setSource): Setup a ScopeGuard to decrement the cursor's request
  count whenever it makes sense to do so.
(WebCore::IDBClient::IDBRequest::dispatchEvent): Clear the ScopeGuard (if it exists) to decrement the count.
(WebCore::IDBClient::IDBRequest::willIterateCursor): Set the ScopeGuard.
(WebCore::IDBClient::IDBRequest::didOpenOrIterateCursor): Clear the ScopeGuard (if it exists) to decrement the count.
* Modules/indexeddb/client/IDBRequestImpl.h:

* Modules/indexeddb/server/UniqueIDBDatabase.cpp:
(WebCore::IDBServer::ScopeGuard::ScopeGuard): Deleted.
(WebCore::IDBServer::ScopeGuard::~ScopeGuard): Deleted.
(WebCore::IDBServer::ScopeGuard::enable): Deleted.
(WebCore::IDBServer::ScopeGuard::disable): Deleted.

* bindings/js/JSIDBCursorWithValueCustom.cpp: Added.
(WebCore::JSIDBCursorWithValue::visitAdditionalChildren):

* platform/ScopeGuard.h: Added.
(WebCore::ScopeGuard::ScopeGuard):
(WebCore::ScopeGuard::~ScopeGuard):
(WebCore::ScopeGuard::enable):
(WebCore::ScopeGuard::disable):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@195997 268f45cc-cd09-0410-ab3c-d52691b4dbfc