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Since ARC code cannot refer to NSAutoreleasePool directly, before this
change block_cleanup_test.h cannot be included from ARC-enabled code.
With this change, the header lists block_cleanup_pool_ as id, which
allows such includes.

BUG=None

Review-Url: https://codereview.chromium.org/2623233002
Cr-Commit-Position: refs/heads/master@{#442927}