[Mac] PasteboardMac.mm build fails

[Mac] PasteboardMac.mm build fails
https://bugs.webkit.org/show_bug.cgi?id=78655

Reviewed by Enrica Casucci.

Source/WebCore:

This patch fixes the code to make a plain text for pasted file names.
The code should return a string of concatenated file names.

Test: editing/pasteboard/drag-files-to-editable-element.html

* platform/mac/PasteboardMac.mm:
(WebCore::Pasteboard::plainText):

LayoutTests:

The added test drops multiple files onto an editable element,
and checks if the filenames are inserted into the editable element.

* editing/pasteboard/drag-files-to-editable-element-expected.txt: Added.
* editing/pasteboard/drag-files-to-editable-element.html: Added.
* platform/chromium/test_expectations.txt:
* platform/efl/Skipped:
* platform/gtk/Skipped:
* platform/qt/Skipped:
* platform/win/Skipped:
* platform/wk2/Skipped:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@107886 268f45cc-cd09-0410-ab3c-d52691b4dbfc