Filter files from dataTransfer.getData on Mac

Filter files from dataTransfer.getData on Mac
https://bugs.webkit.org/show_bug.cgi?id=38876

Reviewed by Enrica Casucci.

Source/WebCore:

The ClipboardMac class now keeps track of whether it was created for copy and paste, dragging and dropping
files or dragging and dropping generic data. This enables the class to block calls to set-/getData when the
clipboard is not operating on generic data, and vice-versa.

Test: fast/events/drop-with-file-paths.html

* editing/mac/EditorMac.mm:
(WebCore::Editor::newGeneralClipboard):
* page/mac/EventHandlerMac.mm:
(WebCore::EventHandler::createDraggingClipboard):
* platform/mac/ClipboardMac.h:
(WebCore::ClipboardMac::create):
(ClipboardMac):
* platform/mac/ClipboardMac.mm:
(WebCore::Clipboard::create):
(WebCore::ClipboardMac::ClipboardMac):
(WebCore::ClipboardMac::getData):
(WebCore::ClipboardMac::setData):
(WebCore::ClipboardMac::files):

LayoutTests:

* fast/events/drop-with-file-paths-expected.txt: Added.
* fast/events/drop-with-file-paths.html: Added.
* fast/events/resources/file-for-drop-with-file-paths.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@113596 268f45cc-cd09-0410-ab3c-d52691b4dbfc