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This CL allows to set different intervals for soft fetches for wifi and
fallback connections. This puts soft fetches on par to persistent
fetches with respect to parameters.

Previously we prioritized soft fetches triggered by NTP_OPENED. This
feature is removed by this CL for the sake of simplicity (NTP_OPENED
is the only trigger that we employ on M57 Stable).

BUG=708146

Review-Url: https://codereview.chromium.org/2794313002
Cr-Commit-Position: refs/heads/master@{#462937}