Fix return type of WTR::buildModifierFlags() in EventSenderProxy.mm. Currently uses `int` where `NSEventModifierFlags` should be used. Regressed back in 2011 with commit 83922@main (r95039).
<rdar://problem/102615921>
Pull request: https://github.com/WebKit/WebKit/pull/6750
Committed 256972@main (99718f07fd9a): <https://commits.webkit.org/256972@main> Reviewed commits have been landed. Closing PR #6750 and removing active labels.