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see go/render-html-multipaste for the design doc.

This is the first CL to render HTML in multipaste.

New objects:
ClipboardHistoryResourceManager
[stub]ClipboardHistoryImageModelFactory/Impl
 - The next CL will include implementation of FactoryImpl.

Changes:
 - ClipboardHistory now has observers which are notified on
   things added/removed from history.
 - ClipboardData is wrapped by ClipboardHistoryItem
   - This has an ID to identify different ClipboardHistoryItems
     quickly.
 - ClipboardHistoryHelper -> ClipboardHistoryResourceManager.
   - CHResourceManager serves as a cache for ImageModels which will
     be rendered by ClipboardImageModelFactory.

Bug: 1117230
Change-Id: Ia73d284eb3481525bc04bb04622801eda3c7a8d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363023
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#800337}