Revert 248259 "[Sync] Remove bookmark model observer on DTC abort"

This breaks ASAN bots.

> [Sync] Remove bookmark model observer on DTC abort
> 
> If the bookmark datatype controller gets aborted, but the bookmark model
> then finishes loading, we crash due to resetting the model load callback
> upon abort. At abort time, we should instead unregister from all model
> callbacks.
> 
> BUG=333948
> 
> Review URL: https://codereview.chromium.org/138543009

TBR=zea@chromium.org

Review URL: https://codereview.chromium.org/137803009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248272 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248272