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downloadTotal is a field that can be set by the developer from the fetch() API, through
BackgroundFetchOptions (https://wicg.github.io/background-fetch/#dictdef-backgroundfetchoptions).

Before this change, the progress bar in the UI showed a percentage based on number of files
downloaded. After this change, the progress bar will use bytes downloaded out of total bytes to be
downloaded to calculate this percentage.

If downloadTotal is set incorrectly (larger than total size of downloads), or not set at all, we
fall back to previous behavior. Additionally, we update the progress bar more often with this
change -- whenever DownloadManager notifies us of more bytes downloaded (~500 ms) instead of only
updating when a download is completed or aborted.

Bug: 813565
Change-Id: Ia704ea99b9ee3966aabb3f64a4ccd2112e36ab80
Reviewed-on: https://chromium-review.googlesource.com/998105
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557243}