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Since the UpdateContext is now a ref counted type, using the iterator
adds an unnecessary level of obfuscation.

Also, to implement task cancellation, a mechanism is needed to relate
the update task and its context, and the iterator is not appropriate
for that.

TBR: waffles
Bug: 822990
Change-Id: I8db51b24e4ef0f9b30f434820fa6d3bd1b066e98
Reviewed-on: https://chromium-review.googlesource.com/966966
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544069}