Before deleting a profile, cancel all in-progress downloads to prevent a "Do you...

Before deleting a profile, cancel all in-progress downloads to prevent a "Do you really want to exit Chrome" popup, which (1) is confusing because the message is not accurate, and (2) prevents a window corresponding to the deleted profile from closing, leaving it in an inconsistent state.

A UI update (warning the user that downloads will be canceled) will follow in a separate CL.

Note: I'm aware that this doesn't address crbug.com/289390 which is another way to trigger the same underlying bug. Working on it...

BUG=336725

Review URL: https://codereview.chromium.org/214523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262970 0039d316-1c4b-4281-b951-d872f2087c98