Revert 267855 "Before this patch set, callback helper can bind p..."
> Before this patch set, callback helper can bind parameters to
> a null function, and the returned function seems to be a callable function.
> If a null function is called, it can crash.
>
>
> This patch makes the callback helper to return a null function
> for a null function bindings.
>
>
> BUG=None
> TEST=./unit_test --gtest_filter="DriveBackendCallbackHelperTest.*"
>
> Review URL: https://codereview.chromium.org/
266823002
TBR=peria@chromium.org
Review URL: https://codereview.chromium.org/
265093003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267879 0039d316-1c4b-4281-b951-d872f2087c98