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This CL adds 3 trace event in background download system to debug
potential jank reporting through crash report.

Bug: 878593
Change-Id: Ibebca6ba331fb00b5b74a327b900c98e8b516151
Reviewed-on: https://chromium-review.googlesource.com/c/1354638Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612356}