Local NTP OneGoogleBar: In case of network errors, keep the cached data

Before, we'd clear the cached data when the refresh request failed due
to e.g. missing network connectivity. Now, we keep the cache in that
case. We still clear the cache if the server responds with an HTTP
error, or when it returns invalid/unexpected data.

Bug: 583292
Change-Id: I974ec774591d77bb11417a7cafdb511934250fcb
Reviewed-on: https://chromium-review.googlesource.com/541357
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481866}
9 files changed