Remove unnecessary dependencies to previews/core

Do not allow data_reduction_proxy/core to depend on previews/core.
This will allow us to move most of the previews/core/* to
previews/content/ in the next CL.

Do not allow cyclic dependency between previews and d_r_p:
Do not allow previews to depend on d_r_p.

Move some of the files from core/ to content/.

There is no functional change in this CL.

Change-Id: Ie5fa660c4880cd965445b44bada5cbd553601685
Bug: 930802
Reviewed-on: https://chromium-review.googlesource.com/c/1462146
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630957}
20 files changed