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Blink currently does this operation on its display item lists to remove
superfluous begin/end pairs. To port Blink's current display item list
operations, such an operation is required.

This is not possible when in "cached picture" mode, but Blink currently only
does this on the unmerged list (in blink::DisplayItemList::add), so this should
be sufficient (if unfortunately asymmetric).

BUG=484943
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1163803003

Cr-Commit-Position: refs/heads/master@{#333084}