Revert of Cleanup viewport scrolling for future scroll customization work.

Revert of Cleanup viewport scrolling for future scroll customization work. (patchset #2 id:20001 of https://codereview.chromium.org/1223713004/)

Reason for revert:
See crbug.com/508509.

We need to fix this case before relanding.

Original issue's description:
> Cleanup viewport scrolling for future scroll customization work.
>
> Viewport scrolling previously returned a struct of information about
> what scrolled. This complicated scroll customization. This patch makes
> |Viewport::ScrollBy| return a single |Vector2dF|, as well as performing
> some other cleanup that will simplify scroll customization.
>
> BUG=505851
> TEST=LayerTreeHostImplTest, WebContentsViewAuraTest.VerticalOverscroll
> CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
>
> Committed: https://crrev.com/3e773d7aeba1eb0077a7907d0f5ff2e72e1c6753
> Cr-Commit-Position: refs/heads/master@{#337694}

TBR=vollick@chromium.org,enne@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=505851
Committed: https://crrev.com/8fe1dd9f7bf1fe50493fc0b9d0300315b127edcd
Cr-Commit-Position: refs/heads/master@{#338071}

[tdresser, 2015-07-09 17:18:18]

Created Revert of Cleanup viewport scrolling for future scroll customization
work.

[commit-bot: I haz the power, 2015-07-09 17:19:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1227823005/1

[commit-bot: I haz the power, 2015-07-09 17:21:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-09 17:22:15]

Patchset 1 (id:??) landed as
https://crrev.com/8fe1dd9f7bf1fe50493fc0b9d0300315b127edcd
Cr-Commit-Position: refs/heads/master@{#338071}