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In this CL, the code of MainButton and DeleteButton is moved to
independent files instead of living in clipboard_history_item_view.cc.
It improves the code readability.

This CL should not introduce any visual differences.

Bug: 1151493, 1152815
Change-Id: I16200f26ca1d5321a1407b6703e297f0b7d7fe89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2556898Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832022}