UMA stats instrumentation for Renderer.AcceleratedFixedRootBackground

UMA stats instrumentation for Renderer.AcceleratedFixedRootBackground

Along with http://crrev.com/23983017, this adds and makes use of
Renderer.AcceleratedFixedRootBackground UMA histogram.

Records three counts:
1. The total number of main frame scrolls.
2. The total number of main frame scrolls with an accelerated fixed root background.
3. The total number of main frame scrolls with an unaccelerated fixed root background.

R=hartmanng@chromium.org,asvitkine@chromium.org
BUG=None

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157457

[hartmanng, 2013-09-09 14:31:26]

https://codereview.chromium.org/23458035/diff/1/Source/core/rendering/RenderL...
File Source/core/rendering/RenderLayerCompositor.cpp (right):

https://codereview.chromium.org/23458035/diff/1/Source/core/rendering/RenderL...
Source/core/rendering/RenderLayerCompositor.cpp:1232: if
(!m_renderView->rootBackgroundIsEntirelyFixed())
I remember you mentioned that pages can have partially-fixed backgrounds - are
you sure "entirely fixed" is the only thing you want to measure?

[Ian Vollick, 2013-09-09 14:35:37]

On 2013/09/09 14:31:26, hartmanng wrote:
>
https://codereview.chromium.org/23458035/diff/1/Source/core/rendering/RenderL...
> File Source/core/rendering/RenderLayerCompositor.cpp (right):
> 
>
https://codereview.chromium.org/23458035/diff/1/Source/core/rendering/RenderL...
> Source/core/rendering/RenderLayerCompositor.cpp:1232: if
> (!m_renderView->rootBackgroundIsEntirelyFixed())
> I remember you mentioned that pages can have partially-fixed backgrounds - are
> you sure "entirely fixed" is the only thing you want to measure?

Yup. We can only accelerate if all background layers are fixed. I'm hoping to
measure the ratio between cases where we could have accelerated and cases where
we actually did.

[hartmanng, 2013-09-09 14:39:15]

lgtm

[Alexei Svitkine (slow), 2013-09-09 14:40:28]

lgtm

[commit-bot: I haz the power, 2013-09-09 14:46:06]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vollick@chromium.org/23458035/1

[commit-bot: I haz the power, 2013-09-09 15:54:14]

Change committed as 157457