Latched scrolling tests are flakey on Mavericks

Latched scrolling tests are flakey on Mavericks
https://bugs.webkit.org/show_bug.cgi?id=133578
<rdar://problem/17180591>

Reviewed by Brent Fulgham.

Tools:
The latched scrolling tests rely on synthetic mousewheel events whose coordinates can
be wrong on machines with more than one screen. This is because we use
[NSScreen mainScreen] when computing the global coordinates of the events, but
[NSScreen mainScreen] returns the screen with the key window, not the first screen.

So replace calls to [NSScreen mainScreen] with [[NSScreen screens] firstObject]. Also
add comments clarifying why the coordinate math works (which is not obvious given
the events have no target window).

Some scrolling tests also dispatched events outside the 800x600 test window, so
fix those tests, and add logging that will appear in test output when this happens.

* DumpRenderTree/mac/DumpRenderTree.mm:
(createWebViewAndOffscreenWindow):
* DumpRenderTree/mac/EventSendingController.mm:
(-[EventSendingController mouseScrollByX:andY:continuously:]):
(-[EventSendingController mouseScrollByX:andY:withWheel:andMomentumPhases:]):
* WebKitTestRunner/mac/EventSenderProxy.mm:
(WTR::EventSenderProxy::mouseMoveTo):
(WTR::EventSenderProxy::mouseScrollBy):
(WTR::EventSenderProxy::continuousMouseScrollBy):
(WTR::EventSenderProxy::mouseScrollByWithWheelAndMomentumPhases):

LayoutTests:
Fix these tests to not dispatch events outside the window, and correct some of the wording
in the tests.

* platform/mac-wk2/tiled-drawing/fast-scroll-div-latched-div-expected.txt:
* platform/mac-wk2/tiled-drawing/fast-scroll-div-latched-div-with-handler-expected.txt:
* platform/mac-wk2/tiled-drawing/fast-scroll-div-latched-div-with-handler.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-div-latched-div.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-div-latched-mainframe-with-handler.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-div-latched-mainframe.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-iframe-latched-mainframe.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-select-latched-mainframe-with-handler.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-select-latched-mainframe.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-select-latched-select-expected.txt:
* platform/mac-wk2/tiled-drawing/fast-scroll-select-latched-select-with-handler-expected.txt:
* platform/mac-wk2/tiled-drawing/fast-scroll-select-latched-select-with-handler.html:
* platform/mac-wk2/tiled-drawing/fast-scroll-select-latched-select.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@169650 268f45cc-cd09-0410-ab3c-d52691b4dbfc