cros: Show rendered HTML in ClipboardHistory

Multiple changes:
 - Show a placeholder image if the ImageModel is not ready yet.
   - In a future CL, we will update the menu real-time. Currently
     if the image is not ready at menu show time, the placeholder
     is used instead.
 - Resize the rendered HTML to match the new menu look.
 - Remove unused vector icons.

Bug: 1120750
Change-Id: I29c5595d7bd5b7330a67b991f842b1150b9614b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401809


Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805892}