When pasting both image data and text into Chrome, use only the text

Summary: Fixes T5437. This actual behavior is debateable but this is the one that seems simplest and most sensible.

Test Plan: Copied some cells from Numbers, pasted them into a remarkup box in Chrome, got just the text.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5437

Differential Revision: https://secure.phabricator.com/D9647