Safari clears selection when its window gets activated via mouse down

Safari clears selection when its window gets activated via mouse down
https://bugs.webkit.org/show_bug.cgi?id=143157

Reviewed by Anders Carlsson.

The bug was caused by PlatformMouseEvent created in WebKit2 not having its event number
even though we were correctly calling setActivationEventNumber in WebPage::acceptsFirstMouse.

Fixed the bug by storing [NSEvent eventNumber] in WebMouseEvent on Mac in WebKit2 as done in WebKit1.

* Shared/WebEvent.h:
(WebKit::WebMouseEvent::eventNumber):
* Shared/WebEventConversion.cpp:
(WebKit::WebKit2PlatformMouseEvent::WebKit2PlatformMouseEvent):
* Shared/WebMouseEvent.cpp:
(WebKit::WebMouseEvent::WebMouseEvent):
(WebKit::WebMouseEvent::encode):
(WebKit::WebMouseEvent::decode):
* Shared/mac/WebEventFactory.mm:
(WebKit::WebEventFactory::createWebMouseEvent):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182087 268f45cc-cd09-0410-ab3c-d52691b4dbfc