[NTP Snippets] Safe(r) defaults for background fetching intervals

components/ntp_snippets/ntp_snippets_service.cc

Index: components/ntp_snippets/ntp_snippets_service.cc
diff --git a/components/ntp_snippets/ntp_snippets_service.cc b/components/ntp_snippets/ntp_snippets_service.cc
index 8ad028bfff07ccd17b6491ef1720c9b36327b1e9..1edb4c8c6e46cd640bebefb9dc73563ea04587f9 100644
--- a/components/ntp_snippets/ntp_snippets_service.cc
+++ b/components/ntp_snippets/ntp_snippets_service.cc
@@ -50,9 +50,9 @@ namespace {
 // histograms with COUNTS() if this number increases beyond 50.
 const int kMaxSnippetCount = 10;
 
-// Default values for snippets fetching intervals.
-const int kDefaultFetchingIntervalWifiChargingSeconds = 30 * 60;
-const int kDefaultFetchingIntervalWifiSeconds = 2 * 60 * 60;
+// Default values for snippets fetching intervals - once per day only.
+const int kDefaultFetchingIntervalWifiChargingSeconds = 0;

[tschumann, 2016/08/25 20:28:52]

it would be really good to document this behavior -- how does this result in
once per day?
Is there any preference implemented to get it do a request over wifi?
is a value of zero deactivating the scheduling? (and what are the implications
on other fetches).

That aside: this also works when the program got kicked out of memory, right?

[Marc Treib, 2016/08/26 08:24:49]

A value of "0" disables the fetching, correct. That behavior is documented on
the NTPSnippetsScheduler interface, though it might make sense to repeat it here
:)

The "once per day" just comes from kDefaultFetchingIntervalFallbackSeconds,
which is not zero.

All these default values can be overridden via Finch.

Yes, this works even when Chrome isn't running. The OS will wake up the device,
start Chrome, and tell it to fetch.

[tschumann, 2016/08/29 12:38:41]

yes, cause when choosing these values you rely on the implementation details, so
better document.
So this means, we don't have a preference to fetch data on wifi-only? 
Now that we have the throttler in place, we can set an upper limit of one
background fetch per day. And then try fetching via wifi every hour and have the
24h fallback in case of no wifi connectivity. WDYT?

+const int kDefaultFetchingIntervalWifiSeconds = 0;
 const int kDefaultFetchingIntervalFallbackSeconds = 24 * 60 * 60;
 
 // Variation parameters than can override the default fetching intervals.