Reduce the number of CachedUAStyles

Reduce the number of CachedUAStyles

CachedUAStyles are large (160b) and not cheap to create because they
include several Length objects which have non-trivial destructors,
making CachedUAStyles expensive to destruct. This patch reduces the
number of CachedUAStyles by only creating them when needed.

There are two changes in this patch:
1) CachedUAStyle& is no longer a member on StyleAdjuster but is
instead passed in through StyleAdjuster::adjustRenderStyle.

2) The cached UA style is now constructed as-needed in
StyleResolverState::cacheUserAgentBorderAndBackground.
StyleResolverState always owned the CachedUAStyle but this patch
makes it explicit by changing the type to OwnPtr<CachedUAStyle>. A
null check was needed in RenderTheme::adjustStyle but otherwise
CachedUAStyle was not accessed without being set.

Before patch:
  facebook.com - 19,850 CachedUAStyles constructed
  wikipedia cat - 22,686 CachedUAStyles constructed

With patch:
  facebook.com - 538 CachedUAStyles constructed
  wikipedia cat - 16 CachedUAStyles constructed

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176163

[abarth-chromium, 2014-06-15 03:44:46]

Seems sensible to me, but I don't hardly know this code.

[pdr., 2014-06-15 03:58:35]

On 2014/06/15 03:44:46, abarth wrote:
> Seems sensible to me, but I don't hardly know this code.

+cc eseidel who added CachedUAStyle
+cc esprehn who knows this area

[esprehn, 2014-06-15 05:10:00]

lgtm, strange, I thought eseidel had made the CachedUAStyle heap allocated a
long time ago.

Do you have numbers on what perf improvement this makes?

https://codereview.chromium.org/338673002/diff/20001/Source/core/rendering/st...
File Source/core/rendering/style/CachedUAStyle.h (right):

https://codereview.chromium.org/338673002/diff/20001/Source/core/rendering/st...
Source/core/rendering/style/CachedUAStyle.h:55: PassOwnPtr<CachedUAStyle>
CachedUAStyle::create(const RenderStyle* style)
Just put this in the class above.

[pdr., 2014-06-15 05:33:46]

Thanks for the review.

I haven't run the numbers but I will update this review once the perfbots come
back.

https://codereview.chromium.org/338673002/diff/20001/Source/core/rendering/st...
File Source/core/rendering/style/CachedUAStyle.h (right):

https://codereview.chromium.org/338673002/diff/20001/Source/core/rendering/st...
Source/core/rendering/style/CachedUAStyle.h:55: PassOwnPtr<CachedUAStyle>
CachedUAStyle::create(const RenderStyle* style)
On 2014/06/15 05:09:59, esprehn wrote:
> Just put this in the class above.

Done.

[pdr., 2014-06-15 05:35:08]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2014-06-15 05:36:12]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/pdr@chromium.org/338673002/40001

[commit-bot: I haz the power, 2014-06-15 07:13:18]

Change committed as 176163