Share more code between iOS and macOS ViewGestureController

Share more code between iOS and macOS ViewGestureController
https://bugs.webkit.org/show_bug.cgi?id=163158

Reviewed by Simon Fraser.

Share canSwipeInDirection() and the (unused on Mac) alternate back-forward list mechanism.
Make ViewGestureController operate in terms of WebPageProxy, not WKWebView,
because it shouldn't know anything about WKWebView.
Refactor scrollEventCanBecomeSwipe a bit to be less repetitive.

* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView setAllowsBackForwardNavigationGestures:]):
* UIProcess/Cocoa/ViewGestureController.cpp:
(WebKit::ViewGestureController::ViewGestureController):
(WebKit::ViewGestureController::setAlternateBackForwardListSourcePage):
(WebKit::ViewGestureController::canSwipeInDirection):
(WebKit::ViewGestureController::gestureControllerForPage): Deleted.
* UIProcess/Cocoa/ViewGestureController.h:
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::WebPageProxy):
* UIProcess/WebPageProxy.h:
(WebKit::WebPageProxy::createWeakPtr):
* UIProcess/ios/ViewGestureControllerIOS.mm:
(WebKit::ViewGestureController::beginSwipeGesture):
(WebKit::ViewGestureController::setAlternateBackForwardListSourceView): Deleted.
(WebKit::ViewGestureController::canSwipeInDirection): Deleted.
* UIProcess/mac/ViewGestureControllerMac.mm:
(WebKit::ViewGestureController::PendingSwipeTracker::PendingSwipeTracker):
(WebKit::ViewGestureController::PendingSwipeTracker::scrollEventCanBecomeSwipe):
(WebKit::ViewGestureController::PendingSwipeTracker::tryToStartSwipe):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@206986 268f45cc-cd09-0410-ab3c-d52691b4dbfc