[Downloads] Allow observers to know when DownloadHistory query is complete.

Introduce OnHistoryQueryComplete() method to DownloadHistory::Observer
which fires when the initial history load is complete.

[email protected]
BUG=590079

Review URL: https://codereview.chromium.org/1833573005

Cr-Commit-Position: refs/heads/master@{#383136}
3 files changed