Simplify some code and rely on TimeDelta::operator/() less.

This code can use TimeDelta::operator%() to express the same thing in a
shorter way.  Add a comment on one less-obvious block.

Bug: none
Change-Id: I2d639ee85b4427a78a902cf8b8c1930828132edc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318967Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791688}

chrome/browser/chromeos/power/ml/recent_events_counter.cc

@@ -44,11 +44,10 @@ void RecentEventsCounter::Log(base::TimeDelta timestamp) {
   }
   first_bucket_index_ = (bucket_index + 1) % num_buckets_;
 
-  int num_cycles = floor(timestamp / duration_);
-  base::TimeDelta cycle_start = num_cycles * duration_;
-  int extra_buckets = floor((timestamp - cycle_start) / bucket_duration_);
-  first_bucket_time_ = cycle_start + extra_buckets * bucket_duration_ +
-                       bucket_duration_ - duration_;
+  // Move the first bucket time such that |bucket_index| is the last bucket in
+  // the period [first_bucket_time_, first_bucket_time_ + duration_).
+  first_bucket_time_ =
+      timestamp - (timestamp % bucket_duration_) + bucket_duration_ - duration_;
 }
 
 int RecentEventsCounter::GetTotal(base::TimeDelta now) {
@@ -73,9 +72,7 @@ int RecentEventsCounter::GetTotal(base::TimeDelta now) {
 int RecentEventsCounter::GetBucketIndex(base::TimeDelta timestamp) const {
   DCHECK_GE(timestamp, base::TimeDelta());
 
-  int num_cycles = floor(timestamp / duration_);
-  base::TimeDelta cycle_start = num_cycles * duration_;
-  int index = floor((timestamp - cycle_start) / bucket_duration_);
+  int index = (timestamp % duration_) / bucket_duration_;
   if (index >= num_buckets_) {
     return num_buckets_ - 1;
   }