Replace EntryArraySync type by an EntrySync[]

Replace EntryArraySync type by an EntrySync[] to match the specification:
http://www.w3.org/TR/file-system-api/#widl-DirectoryReaderSync-readEntries-EntrySync

As a consequence, regular JavaScript Array methods can now be used.
There is no longer an item() method exposed but developers usually
access items using [] operator.

BUG=268356

Review URL: https://chromiumcodereview.appspot.com/22673003

git-svn-id: svn://svn.chromium.org/blink/trunk@155869 bbb929c8-8fbe-4397-9dbb-9b2b20218538