IDBTransaction / IDBObjectStore should not prevent a page from entering the back...

IDBTransaction / IDBObjectStore should not prevent a page from entering the back / forward cache
https://bugs.webkit.org/show_bug.cgi?id=202291
<rdar://problem/55760106>

Reviewed by Geoffrey Garen.

Source/WebCore:

IDBObjectStore does not fire any events and should therefore not cause any problem while in PageCache.
IDBTransaction can only fire abort/complete/error events, all of which are being enqueued to the
DocumentEventQueue. The DocumentEventQueue subclasses SuspendableTimer which will correctly suspend
while the document is in page cache. IDBTransaction will therefore not cause events to fire (and JS
to run) while in page cache.

Test: storage/indexeddb/IDBTransaction-page-cache.html

* Modules/indexeddb/IDBObjectStore.cpp:
(WebCore::IDBObjectStore::canSuspendForDocumentSuspension const):
* Modules/indexeddb/IDBTransaction.cpp:
(WebCore::IDBTransaction::canSuspendForDocumentSuspension const):

LayoutTests:

Add layout test coverage.

* storage/indexeddb/IDBTransaction-page-cache.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@250531 268f45cc-cd09-0410-ab3c-d52691b4dbfc