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This CL moves PerformBatchOperations() out of the responsibility of
WebStateList serialization and into the caller's responsibility.

This allows callers to define what a batch operation is. For example,
when restoring closed tabs, a batch operation includes modifying
TabRestoreService at the same time as adding WebStates to the WebStateList.

Note: PerformBatchOperation was recently added to web_state_list_serialization
in this CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1865161

Bug: 1014526
Change-Id: I40b11e4fe9b30a5fe18e0e8e404c298b04676d1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867389
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707387}