Add DownloadHooks as a no-op function to git_cl to unblock android bisectors.
Add DownloadHooks as a no-op function to git_cl to unblock android bisectors.

BUG=600473

Review URL: https://codereview.chromium.org/1859873002

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@299688 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools