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To be consistent with the rest of the artifacts in the component updater,
the non-blocking calls for the CrxDownloader take a callback to be called
when the non-blocking call completes.

This change allows adding further callbacks besides the completion
callback, such as a callback to be called when download progress is made.

This change is a pure refactoring and it should be a NOP in terms of
existing functionality.

BUG=366388

Review URL: https://codereview.chromium.org/250383008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266304 0039d316-1c4b-4281-b951-d872f2087c98