capping how many times background download resumption can automatically start

If download gets interrupted due to browser kill, we schedule a browser
restart.
And we could stuck in a cycle of kill->restart->kill.
This CL caps the number of auto resumptions.
And reset the cap when user relaunches chrome.

BUG=623242

Review-Url: https://codereview.chromium.org/2094033002
Cr-Commit-Position: refs/heads/master@{#402340}
3 files changed