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Recently we discovered, that our dismissed IDs pruning may happen at a
wrong time, when DownloadManager does not have a complete list.
Unfortunately, there is no way to check whether DownloadManager
finished starting up.
Therefore, this CL removes pruning of asset downloads and instead
limits maximal number of stored IDs to 100 (the oldest are removed
once the threshold is reached).

BUG=672758

Review-Url: https://codereview.chromium.org/2562073002
Cr-Commit-Position: refs/heads/master@{#437882}