Expose a constructor for PointerEvent from PlatformTouchEvent

Expose a constructor for PointerEvent from PlatformTouchEvent
https://bugs.webkit.org/show_bug.cgi?id=191238
<rdar://problem/45795682>

Reviewed by Antoine Quint.

Implement a constructor that takes a PlatformTouchEvent to
create a PointerEvent. At the moment the code to call this
constructor will live in WebKitAdditions, so no new tests
yet.

* SourcesCocoa.txt: Add PointerEventIOS.cpp.
* WebCore.xcodeproj/project.pbxproj: Ditto.
* dom/EventNames.h: Add macros for pointerdown, pointermove,
     pointerup, pointercancel.
* dom/PointerEvent.cpp: Remove JSC namespace.
* dom/PointerEvent.h: Add create and constructor that takes
     a PlatformTouchEvent.
* dom/ios/PointerEventIOS.cpp: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@237816 268f45cc-cd09-0410-ab3c-d52691b4dbfc