Add events to represent the start/end of a gesture scroll

Add events to represent the start/end of a gesture scroll
https://bugs.webkit.org/show_bug.cgi?id=53215

Reviewed by Maciej Stachowiak.

Source/JavaScriptCore:

* wtf/Platform.h: Add ENABLE for gesture events.

Source/WebCore:

* WebCore.exp.in:
Add new file.

* WebCore.xcodeproj/project.pbxproj:
Add new file.

* page/EventHandler.cpp:
(WebCore::EventHandler::handleGestureEvent):
* page/EventHandler.h:
Add entry point for handling gesture events.

* platform/PlatformGestureEvent.h: Added.
(WebCore::PlatformGestureEvent::PlatformGestureEvent):
(WebCore::PlatformGestureEvent::type):
(WebCore::PlatformGestureEvent::position):
(WebCore::PlatformGestureEvent::globalPosition):
(WebCore::PlatformGestureEvent::timestamp):
Add platform agnostic representation of a gesture event.

Source/WebKit2:

* Scripts/webkit2/messages.py:
* Shared/WebEvent.h:
(WebKit::WebGestureEvent::WebGestureEvent):
(WebKit::WebGestureEvent::position):
(WebKit::WebGestureEvent::globalPosition):
* Shared/WebEventConversion.cpp:
(WebKit::WebKit2PlatformGestureEvent::WebKit2PlatformGestureEvent):
(WebKit::platform):
* Shared/WebEventConversion.h:
* Shared/WebGestureEvent.cpp: Added.
(WebKit::WebGestureEvent::WebGestureEvent):
(WebKit::WebGestureEvent::encode):
(WebKit::WebGestureEvent::decode):
(WebKit::WebGestureEvent::isGestureEventType):
* Shared/mac/WebEventFactory.h:
* Shared/mac/WebEventFactory.mm:
(WebKit::gestureEventTypeForEvent):
(WebKit::WebEventFactory::createWebGestureEvent):
* UIProcess/API/mac/WKView.mm:
(-[WKView shortCircuitedEndGestureWithEvent:]):
(-[WKView beginGestureWithEvent:]):
(-[WKView viewDidMoveToWindow]):
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::handleGestureEvent):
(WebKit::WebPageProxy::didReceiveEvent):
* UIProcess/WebPageProxy.h:
* WebKit2.xcodeproj/project.pbxproj:
* WebProcess/WebPage/WebPage.cpp:
(WebKit::handleGestureEvent):
(WebKit::WebPage::gestureEvent):
* WebProcess/WebPage/WebPage.h:
* WebProcess/WebPage/WebPage.messages.in:
Pipe gesture events down to WebCore.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@76745 268f45cc-cd09-0410-ab3c-d52691b4dbfc