Minor changes to download protection to help diagnose crash

1. Add defensive check in DownloadProtectionService, in case it's being
deleted before the CheckClientDownloadRequest finishes.
2. Don't set item_ to nullptr anymore, always call RemoveObserver at
destruction. This will ensure that the destructor for
CheckClientDownloadRequest isn't being destroyed before the DownloadItem.

Bug: 915037
Change-Id: Ibd1ce1d2f11bee174b399a850ee86aa4a2808268
Reviewed-on: https://chromium-review.googlesource.com/c/1378853
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Nathan Parker <nparker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616924}
1 file changed