Get rid of AXComputedObjectAttributeCache

Get rid of AXComputedObjectAttributeCache

We added AXComputedObjectAttributeCache to WebKit because we wanted to
cache some expensive computations in the AX tree, but none of the other
WebKit ports wanted it.

A much simpler solution is to just store the cached value inside the
AXObject. To determine if the value is out of date, I added a
modification count that increments whenever there's a layout or DOM
mutation. We can reuse the same modification count for more cached
attributes.

BUG=425879
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=184331

[dmazzoni, 2014-10-22 06:22:38]

dmazzoni@chromium.org changed reviewers:
+ aboxhall@chromium.org

[dmazzoni, 2014-10-23 15:44:52]

Ping

[aboxhall, 2014-10-23 21:48:11]

lgtm

https://codereview.chromium.org/670093002/diff/1/Source/core/accessibility/AX...
File Source/core/accessibility/AXObject.h (right):

https://codereview.chromium.org/670093002/diff/1/Source/core/accessibility/AX...
Source/core/accessibility/AXObject.h:584: // The following cached attribute
values are only valid if m_lastModificationCount matches
AXObjectCacheImpl::modificationCount().
What does "the following cached attribute values" here refer to?

Oh right, just m_cachedIsIgnored. Perhaps this could be reworded slightly more
clearly, something like
"The cached attribute values starting with m_cached* are only valid if
m_lastModificationCount matches ..."

[dmazzoni, 2014-10-23 21:51:52]

https://codereview.chromium.org/670093002/diff/1/Source/core/accessibility/AX...
File Source/core/accessibility/AXObject.h (right):

https://codereview.chromium.org/670093002/diff/1/Source/core/accessibility/AX...
Source/core/accessibility/AXObject.h:584: // The following cached attribute
values are only valid if m_lastModificationCount matches
AXObjectCacheImpl::modificationCount().
On 2014/10/23 21:48:11, aboxhall wrote:
> What does "the following cached attribute values" here refer to?
> 
> Oh right, just m_cachedIsIgnored. Perhaps this could be reworded slightly more
> clearly, something like
> "The cached attribute values starting with m_cached* are only valid if
> m_lastModificationCount matches ..."

Sure. In the following changes I'm hoping to add several more.

[dmazzoni, 2014-10-24 05:46:39]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2014-10-24 05:47:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/670093002/20001

[commit-bot: I haz the power, 2014-10-24 06:51:12]

Committed patchset #2 (id:20001) as 184331