metrics: reduce perf periodic collection interval to 3 hours from 24

This should increase the quantity of perf profiles collected for
chromiumos-wide-profiling by a factor of 8. Based on the current amount of
data received, this should increase the number of profiles collected to a
quantity less than it was before the collection scheduling was changed in
https://codereview.chromium.org/282093011/

TEST=Not really... This is just turning a knob. I did compile it.
BUG=chromium:413951

Review URL: https://codereview.chromium.org/559843007

Cr-Commit-Position: refs/heads/master@{#295154}

chrome/browser/metrics/perf_provider_chromeos.cc

@@ -28,8 +28,7 @@ namespace {
 
 // Partition time since login into successive intervals of this size. In each
 // interval, pick a random time to collect a profile.
-// This interval is twenty-four hours.
-const size_t kPerfProfilingIntervalMs = 24 * 60 * 60 * 1000;
+const size_t kPerfProfilingIntervalMs = 3 * 60 * 60 * 1000;
 
 // Default time in seconds perf is run for.
 const size_t kPerfCommandDurationDefaultSeconds = 2;