Revert of Clip scrollbars to box bounds when they don't fit.

Revert of Clip scrollbars to box bounds when they don't fit. (patchset #6 id:180001 of https://codereview.chromium.org/1448253002/ )

Reason for revert:
Speculative revert for various test failures, e.g. http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29%2832%29/builds/22662/steps/browser_tests/logs/stdio and http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%281%29/builds/43345/steps/browser_tests/logs/stdio

Original issue's description:
> Clip scrollbars to box bounds when they don't fit.
>
> There are three cases:
>
> (1) scroller is a non-composited self-painting layer
> (2) scroller is not a self-painting layer
> (3) scroller is a composited layer
> (3a) scroller is a composited layer that does not use composited scrolling
>
> Case (1) already worked correctly through the background clip applied by
> PaintLayerPainter.  Case (2) is addressed by the change in BlockPainter.
>
> Case (3) is addressed by the changes in CompositedLayerMapping.  The clip is
> applied by m_overflowControlsHostLayer.  GraphicsLayerTreeBuilder is modified
> to preserve the layer tree in case (3a).
>
> I have renamed m_overflowControlsClippingLayer for clarity, since the overflow
> controls can now be clipped for two different reasons.
>
> Note that for an iframe with CSS "resize", the scroll corner comes from the
> iframe element's CompositedLayerMapping, not the inner PaintLayerCompositor.
> For this reason, PLC::attachFrameContentLayersToIframeLayer must preserve
> overflow controls just like the "!parented" path of GraphicsLayerTreeBuilder.
>
> BUG=549174
>
> Committed: https://crrev.com/bf0a5ac073c0844a514a979beab5c8b08a445393
> Cr-Commit-Position: refs/heads/master@{#360860}

TBR=chrishtr@chromium.org,vollick@chromium.org,skobes@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=549174
Committed: https://crrev.com/336d7bf63199b6681cf5c8d2789ef06cd3bb8994
Cr-Commit-Position: refs/heads/master@{#360913}

[grt (UTC plus 2), 2015-11-20 20:53:34]

Created Revert of Clip scrollbars to box bounds when they don't fit.

[commit-bot: I haz the power, 2015-11-20 20:53:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1457963006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1457963006/1

[commit-bot: I haz the power, 2015-11-20 20:56:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-20 20:56:51]

Patchset 1 (id:??) landed as
https://crrev.com/336d7bf63199b6681cf5c8d2789ef06cd3bb8994
Cr-Commit-Position: refs/heads/master@{#360913}