Moved the FakeDownloadItem to content/public/test.

The new tab page tests implemented a mostly-complete FakeDownloadItem.
content/public/test already contains a mock for a DownloadItem and Fakes
for other classes, and the BackgroundFetch tests will also be needing
a fake DownloadItem, so this CL moves the FakeDownloadItem to the more
accessible location.

BUG=692621

Review-Url: https://codereview.chromium.org/2745003003
Cr-Commit-Position: refs/heads/master@{#456394}
6 files changed