[iOS][wk2] Swiping back briefly shows the previous page before loading the new one

[iOS][wk2] Swiping back briefly shows the previous page before loading the new one
https://bugs.webkit.org/show_bug.cgi?id=133885

Reviewed by Simon Fraser.

Remove a race between the UI and Web processes when removing the swipe snapshot.
Previously, it was possible to get a commit from the Web process with layer content
(and render tree size) from the previous page *after* sending the navigation request
to the page, because of the asynchronicity of layer tree commits. This could cause
the snapshot to be removed early (if the previous fully-loaded page had a sufficiently
large render tree size), revealing the old tiles underneath the snapshot.

* Shared/mac/RemoteLayerTreeTransaction.h:
(WebKit::RemoteLayerTreeTransaction::transactionID):
(WebKit::RemoteLayerTreeTransaction::setTransactionID):
* Shared/mac/RemoteLayerTreeTransaction.mm:
(WebKit::RemoteLayerTreeTransaction::encode):
(WebKit::RemoteLayerTreeTransaction::decode):
* UIProcess/DrawingAreaProxy.h:
(WebKit::DrawingAreaProxy::lastVisibleTransactionID):
* UIProcess/mac/RemoteLayerTreeDrawingAreaProxy.h:
* UIProcess/mac/RemoteLayerTreeDrawingAreaProxy.mm:
(WebKit::RemoteLayerTreeDrawingAreaProxy::RemoteLayerTreeDrawingAreaProxy):
(WebKit::RemoteLayerTreeDrawingAreaProxy::commitLayerTree):
(WebKit::RemoteLayerTreeDrawingAreaProxy::coreAnimationDidCommitLayers):
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.h:
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::RemoteLayerTreeDrawingArea):
(WebKit::RemoteLayerTreeDrawingArea::flushLayers):
(WebKit::RemoteLayerTreeDrawingArea::didUpdate):
Keep track of an ever-increasing transaction ID in RemoteLayerTreeDrawingArea(Proxy).
It increments in the UI process at didUpdate time, because the Web process cannot
have started on a new layer tree commit until didUpdate is sent.
It increments in the Web process at commit time.

* UIProcess/ios/ViewGestureControllerIOS.mm:
(WebKit::ViewGestureController::endSwipeGesture):
(WebKit::ViewGestureController::setRenderTreeSize):
* UIProcess/mac/ViewGestureController.h:
Adopt transaction IDs; don't remove the snapshot until the commit
that includes the navigation arrives.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170035 268f45cc-cd09-0410-ab3c-d52691b4dbfc