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Files app copies raw images onto the clipboard by:
1. In JS, save files app metadata.
2. Later, on the chrome side, add raw image data by:
  - Reading step 1's data from Clipboard.
  - Add raw image data.
  - Write to clipboard a second time.

In a way, files app uses the clipboard as a temp variable.

This results in two entries to ClipboardHistory.
Delete the old one, then allow the second Write to create
a new entry in history.

Bug: 1150155
Change-Id: I90649d9b56ebfc538ab962e4c769a869b9d5d082
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630405Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844272}