cc: Add a rails argument to InputHandler::ScrollBy

cc: Add a rails argument to InputHandler::ScrollBy

This had previously been left as a TODO in InputHandlerProxy in
work that had needed to be merged.

BUG=468454

[ccameron, 2015-04-02 16:21:21]

ccameron@chromium.org changed reviewers:
+ aelias@chromium.org

[ccameron, 2015-04-02 16:21:46]

Paying down technical debt from M42.

[aelias_OOO_until_Jul13, 2015-04-02 17:51:56]

lgtm

[enne (OOO), 2015-04-02 19:45:54]

lgtm as well

[ccameron, 2015-04-02 19:52:43]

Thanks!

jdduke is suggesting in https://codereview.chromium.org/1052433003/ that this
may not be the way to go -- maybe rather than just saying "rails mode is
vertical", we should send a "deltaX with rails filter applied" as a separate
field, and always use that, rather than doing a "well, if the rails mode is
vertical than set X to zero.

Does that seem better?

[aelias_OOO_until_Jul13, 2015-04-02 20:59:58]

aelias@chromium.org changed reviewers:
+ enne@chromium.org

[aelias_OOO_until_Jul13, 2015-04-02 20:59:59]

It might be -- I haven't made up my mind, I'm trying to wrap my head around
rails customization right now.