Implement FileSystemDirectoryEntry.getDirectory()

Implement FileSystemDirectoryEntry.getDirectory()
https://bugs.webkit.org/show_bug.cgi?id=176168
<rdar://problem/34187787>

Reviewed by Darin Adler.

Source/WebCore:

Implement FileSystemDirectoryEntry.getDirectory() as per:
- https://wicg.github.io/entries-api/#dom-filesystemdirectoryentry-getdirectory

Test: editing/pasteboard/datatransfer-items-drop-getDirectory.html

* Modules/entriesapi/DOMFileSystem.cpp:
(WebCore::isValidVirtualPath):
(WebCore::resolveRelativeVirtualPath):
(WebCore::DOMFileSystem::getEntry):
* Modules/entriesapi/DOMFileSystem.h:
* Modules/entriesapi/FileSystemDirectoryEntry.cpp:
(WebCore::FileSystemDirectoryEntry::getEntry):
(WebCore::FileSystemDirectoryEntry::getFile):
(WebCore::FileSystemDirectoryEntry::getDirectory):
* Modules/entriesapi/FileSystemDirectoryEntry.h:

LayoutTests:

Add layout test coverage.

* editing/pasteboard/datatransfer-items-drop-getDirectory-expected.txt: Added.
* editing/pasteboard/datatransfer-items-drop-getDirectory.html: Added.
* platform/wk2/TestExpectations:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@221540 268f45cc-cd09-0410-ab3c-d52691b4dbfc