Don't attempt to clone NSPasteboards.

It doesn't work, as any file promises will be called in, and the
loop over all the flavors will not be equipped to handle it.

The MacViews code wants to provide an OSExchangeDataProvider for
drags that it didn't initiate, so wrap a provider around the
pasteboard rather than clone it.

This is a third attempt to land this (previously attempted
with e11a5a297736acdbc93609906facd5da04e7b6c0 and with
7b01a37f8c4b5fb80e084bd1174d12bf39fec3a6). This version fixes
extension drags.

BUG=908349, 928684, 932051

Change-Id: Ie144ddf78c7e95f6946bbbe77bbda7027914761d
Reviewed-on: https://chromium-review.googlesource.com/c/1476239
Auto-Submit: Avi Drissman <avi@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633531}
6 files changed