[Feed] Break offline pages ties by namespace first.

Bug: 888767
Change-Id: Iaac4393ba4609095b58d00dbcfb1234e20296a0e
Reviewed-on: https://chromium-review.googlesource.com/c/1279045
Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600089}
2 files changed