cc: Fix and simplify DelayBasedTimeSource

cc: Fix and simplify DelayBasedTimeSource

- This converts some floating point math to integer math
to avoid rounding errors.

- Assigns last_tick_time_ to be the time it was *supposed*
to tick and no longer uses it as a synonym for "now".

- Adds trace events to help debug when the interval or
phase changes.

BUG=256650
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=210101

[brianderson, 2013-07-02 23:56:33]

Turns out DBTS is causing some flake on the bots, most likely because of
rounding errors that cause us to hit a DCHECK. This patch attempts to fix the
flake, but also adds two other improvements.

[brianderson, 2013-07-03 00:01:54]

https://codereview.chromium.org/18589002/diff/1/cc/scheduler/delay_based_time...
File cc/scheduler/delay_based_time_source.cc (right):

https://codereview.chromium.org/18589002/diff/1/cc/scheduler/delay_based_time...
cc/scheduler/delay_based_time_source.cc:21: static const int kDoubleTickDivisor
= 4;
We can divide TimeTicks so I switched this to a divisor to avoid floating point
math.

https://codereview.chromium.org/18589002/diff/1/cc/scheduler/delay_based_time...
cc/scheduler/delay_based_time_source.cc:90: last_tick_time_ =
current_parameters_.tick_target;
Instead of setting the last_tick_time_ to the time at which this task happened
to be scheduled, this will return the ideal tick time giving us a less jittery
timestamp.

https://codereview.chromium.org/18589002/diff/1/cc/scheduler/delay_based_time...
cc/scheduler/delay_based_time_source.cc:142: TRACE_EVENT0("cc",
"DelayBasedTimeSource::PhaseChanged");
IntervalChanged and PhaseChanged show up way too often on some platforms,
causing unnecessary jank. See
https://code.google.com/p/chromium/issues/detail?id=252087. This will make it
easier to notice and fix.

https://codereview.chromium.org/18589002/diff/1/cc/scheduler/delay_based_time...
cc/scheduler/delay_based_time_source.cc:209: int intervals_elapsed = (now -
next_parameters_.tick_target) / new_interval;
Goodbye floating point math.

https://codereview.chromium.org/18589002/diff/1/cc/scheduler/delay_based_time...
cc/scheduler/delay_based_time_source.cc:224: if (new_tick_target -
last_tick_time_ <= new_interval / kDoubleTickDivisor)
Goodbye more floating point math.

[nduca, 2013-07-03 05:37:25]

lgtm

https://codereview.chromium.org/18589002/diff/10001/cc/scheduler/delay_based_...
File cc/scheduler/delay_based_time_source.cc (right):

https://codereview.chromium.org/18589002/diff/10001/cc/scheduler/delay_based_...
cc/scheduler/delay_based_time_source.cc:106: TRACE_EVENT0("cc",
"DelayBasedTimeSource::IntervalChanged");
I'd suggest using an instant event with thread scope. If that doesn't show up
enough in the UI, remind me tomorrow and I'll try to fix it.

[nduca, 2013-07-03 05:37:56]

the instant comment applies to all the events you issue, i think...

[commit-bot: I haz the power, 2013-07-03 17:50:29]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/brianderson@chromium.org/18589002/23001

[commit-bot: I haz the power, 2013-07-04 02:03:20]

Change committed as 210101