Implement one-page-at-a-time mousewheel scrolling in the impl thread.

Implement one-page-at-a-time mousewheel scrolling in the impl thread.

Previously the scrollByPage setting in the incoming scroll event was getting
ignored.  

BUG=160122

[enne (OOO), 2013-01-16 17:26:03]

What about one-pixel-at-a-time? Aren't there three states to that enum?

[jamesr, 2013-03-06 23:50:37]

The logic to do scrollByPage in WebCore seems to be different.  Looking at
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...,
it's using a few pieces of data that I don't see here such as
Scrollbar::minFractionToStepWhenPaging() and
Scrollbar::maxOverlapBetweenPages().  Are those here somewhere that I don't see?

https://codereview.chromium.org/11975007/diff/1/cc/layer_tree_host_impl.cc
File cc/layer_tree_host_impl.cc (right):

https://codereview.chromium.org/11975007/diff/1/cc/layer_tree_host_impl.cc#ne...
cc/layer_tree_host_impl.cc:1278: pendingDelta = gfx::ScaleVector2d(pendingDelta,
visibleScrollArea.width(), visibleScrollArea.height());
This seems a bit strange.  scrollByPage is only valid for mousewheels and wheels
can't be diagonal.

https://codereview.chromium.org/11975007/diff/1/webkit/compositor_bindings/we...
File webkit/compositor_bindings/web_layer_impl.h (right):

https://codereview.chromium.org/11975007/diff/1/webkit/compositor_bindings/we...
webkit/compositor_bindings/web_layer_impl.h:82: virtual void
setVisibleScrollArea(WebSize) OVERRIDE;
We already have this information in cc (We need it in order to render
correctly).  Why the new API?

[vangelis, 2013-03-07 02:01:21]

On 2013/01/16 17:26:03, enne wrote:
> What about one-pixel-at-a-time? Aren't there three states to that enum?

Back from the dead... :)

Sorry I'm not sure which enum you're referring to. The WebInputEvent we get only
has a boolean that determines whether to scroll-by-page. Also, at least on
Windows the only available options for scrolling are scroll by a certain number
of lines and scroll by a screen-full.

[jamesr, 2013-03-07 03:02:50]

The determination of whether a wheel is pixel-accurate or represents logical
lines happens earlier, in computing the deltaX or deltaY.

[enne (OOO), 2013-04-01 16:20:30]

Closing this code review, as jamesr's https://codereview.chromium.org/12582009
implemented this.