aura: Populate and use the newly added acceleration ratio fields in wheel-events.

aura: Populate and use the newly added acceleration ratio fields in wheel-events.

Wheel-scroll event deltas can have acceleration (e.g. when doing repeated
scrolls). But for overscroll, it makes more sense to use the unaccelerated
deltas.

BUG=175604
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=182473

[sadrul, 2013-02-13 16:46:14]

The webkit patch this depends on: http://trac.webkit.org/changeset/142752

[rjkroege, 2013-02-13 18:44:31]

I think that it would be safer/better to ship the unaccelerated deltas instead
of the ratio. My reasoning is that as it is: you are dividing by a field that
you will now need to make sure is non-zero everywhere... like on mac. or Linux.
Which it doesn't look like you've done?

[sadrul, 2013-02-13 19:08:56]

On 2013/02/13 18:44:31, rjkroege wrote:
> I think that it would be safer/better to ship the unaccelerated deltas instead
> of the ratio. My reasoning is that as it is: you are dividing by a field that
> you will now need to make sure is non-zero everywhere... like on mac. or
Linux.
> Which it doesn't look like you've done?

The ratios by default are set to 1. So we don't need to set it unless we have
enough information about the acceleration (and we have this info only for aura
trackpad events). This was one of the appealing bits of having an acceleration
ratio, instead of the unaccelerated deltas (the unaccelerated deltas would have
to always be initialized by platform-specific event-conversion code, while the
acceleration ratio needs to be initialized only when there is enough information
to compute the acceleration).

I initially went with the unaccelerated deltas (patchset1 in this CL), but
abarth@ proposed using ratios, and I switched to that since that turns out to be
simpler (https://bugs.webkit.org/show_bug.cgi?id=109611#c6)

[rjkroege, 2013-02-13 22:08:03]

lgtm

https://codereview.chromium.org/12223110/diff/12001/content/browser/renderer_...
File content/browser/renderer_host/overscroll_controller.cc (right):

https://codereview.chromium.org/12223110/diff/12001/content/browser/renderer_...
content/browser/renderer_host/overscroll_controller.cc:175:
ProcessOverscroll(wheel.deltaX / wheel.accelerationRatioX,
nit: inverse ratios (i.e. multiplies are cheaper)

[rjkroege, 2013-02-14 15:59:55]

still lgtm