Create a synthetic field trial for precache

Create a synthetic field trial for precache

Registers the precache synthetic field trial for users whom the precache
task started in the last 15 days.

BUG=663987

[Raj, 2016-12-08 06:43:12]

The CQ bit was checked by rajendrant@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-12-08 06:43:30]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Raj, 2016-12-08 06:46:59]

rajendrant@chromium.org changed reviewers:
+ twifkak@chromium.org

[Raj, 2016-12-08 06:47:00]

ptal

[commit-bot: I haz the power, 2016-12-08 07:46:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-12-08 07:46:18]

Dry run: This issue passed the CQ dry run.

[twifkak, 2016-12-08 21:49:22]

lgtm

https://codereview.chromium.org/2562573003/diff/1/chrome/browser/precache/pre...
File chrome/browser/precache/precache_util.cc (right):

https://codereview.chromium.org/2562573003/diff/1/chrome/browser/precache/pre...
chrome/browser/precache/precache_util.cc:88:
ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial(
Ooh, how about a multi-group field trial? Just learned about it now. It occurs
to me the next question we will ask is "What about only people who precached
yesterday?"

Some proof-of-concept code:

const size_t kDaysForRecentPrecacheCandidates = 1;

std::vector<uint32_t> groups;
base::TimeDelta time_ago = base::Time::Now() - last_precache_time;
if (time_ago <= base::TimeDelta::FromDays(kDaysForPrecacheCandidates))
  groups.push_back(metrics::HashName(group_name);
if (time_ago <=
    base::TimeDelta::FromDays(kDaysForRecentPrecacheCandidates))
  groups.push_back(metrics::HashName(group_name + "Recent");
RegisterSyntheticMultiGroupFieldTrial(
  kPrecacheSyntheticFieldTrialName, groups);

https://codereview.chromium.org/2562573003/diff/1/chrome/browser/precache/pre...
File chrome/browser/precache/precache_util.h (right):

https://codereview.chromium.org/2562573003/diff/1/chrome/browser/precache/pre...
chrome/browser/precache/precache_util.h:20: // started in the last
|kDaysForPrecacheCandidates| days. The group name  of the
s/name  of/name of/

https://codereview.chromium.org/2562573003/diff/1/components/precache/content...
File components/precache/content/precache_manager.cc (right):

https://codereview.chromium.org/2562573003/diff/1/components/precache/content...
components/precache/content/precache_manager.cc:68:
BrowserThread::PostTaskAndReplyWithResult(
Good idea, sticking it in the PrecacheManager constructor.

https://codereview.chromium.org/2562573003/diff/1/components/precache/content...
File components/precache/content/precache_manager_unittest.cc (right):

https://codereview.chromium.org/2562573003/diff/1/components/precache/content...
components/precache/content/precache_manager_unittest.cc:128: void
RegisterSyntheticFieldTrial(const base::Time& last_precache_time) {}
Would be nice to add a small check that this is called. e.g. inside
PrecacheManagerUnderTest, add:

MOCK_METHOD1(RegisterSyntheticFieldTrial,
             void(const base::Time& last_precache_time));

and then initialize with that, and add the appropriate EXPECT_CALL to
StartAndFinishPrecaching.

[Raj, 2016-12-08 22:06:01]

The CQ bit was checked by rajendrant@chromium.org

[commit-bot: I haz the power, 2016-12-08 22:06:37]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Raj, 2016-12-08 22:15:20]

The CQ bit was unchecked by rajendrant@chromium.org

[twifkak, 2016-12-13 23:09:03]

Description was changed from

==========
Create a synthetic field trial for precache

Registers the precache synthetic field trial for users whom the precache
task started in the last 15 days.

BUG=663987
==========

to

==========
Create a synthetic field trial for precache

Registers the precache synthetic field trial for users whom the precache
task started in the last 15 days.

BUG=663987
==========

[twifkak, 2016-12-13 23:09:03]

twifkak@chromium.org changed reviewers:
+ rkaplow@chromium.org

[twifkak, 2016-12-13 23:09:42]

rkaplow: LG needed for chrome/browser/metrics/chrome_metrics_service_accessor.h.
(Plus you may be interested in the rest.)

[rkaplow, 2016-12-16 17:18:58]

were you planning on addressing the suggestions on this CL?