Cache and reuse the NodeList returned by Node::childNodes().

Cache and reuse the NodeList returned by Node::childNodes().
<http://webkit.org/b/76591>

Reviewed by Ryosuke Niwa.

Source/WebCore:

Instead of only caching the DynamicNodeList::Caches for .childNodes on NodeRareData,
cache the full ChildNodeList object. Lifetime management is left to wrappers who
invalidate the cached (raw) pointer via Node::removeCachedChildNodeList(), called
from ~ChildNodeList().

This is a slight behavior change, in that Node.childNodes === Node.childNodes will
now be true. This matches the behavior of both Firefox and Opera.

This reduces memory consumption by 192 kB (on 32-bit) when viewing the full
HTML5 spec at <http://whatwg.org/c>

Test: fast/dom/gc-9.html
      fast/dom/node-childNodes-idempotence.html

* dom/Node.cpp:
(WebCore::Node::childNodes):
* dom/NodeRareData.h:
(WebCore::NodeRareData::NodeRareData):
(WebCore::NodeRareData::childNodeList):
(WebCore::NodeRareData::setChildNodeList):

    Only construct one ChildNodeList per Node and store it on NodeRareData for
    retrieval across childNodes() calls.

* dom/ChildNodeList.h:
(WebCore::ChildNodeList::create):
* dom/ChildNodeList.cpp:
(WebCore::ChildNodeList::ChildNodeList):

    Construct the Caches at creation instead of passing it to the constructor.

(WebCore::ChildNodeList::reset):

    Added, resets the internal cache.

(WebCore::ChildNodeList::~ChildNodeList):

    Call Node::removeCachedChildNodeList().

* dom/DynamicNodeList.cpp:
* dom/DynamicNodeList.h:

    Have DynamicNodeList (and subclasses) respond "true" to isDynamicNodeList().
    Previously only DynamicSubtreeNodeList (and subclasses) were doing this.
    Without it, JSC may GC our ChildNodeLists prematurely (due to NodeList's
    isReachableFromOpaqueRoots() implementation checking isDynamicNodeList().)

* dom/Node.h:
* dom/Node.cpp:
(WebCore::Node::removeCachedChildNodeList):

    Added for ~ChildNodeList() to remove the pointer to itself from the Node.

(WebCore::NodeRareData::clearChildNodeListCache):

    Call ChildNodeList::reset().

LayoutTests:

Updated gc-9.html to document the new lifetime characteristics of a .childNodes with
custom properties. Also added a test to verify that .childNodes === .childNodes.

* fast/dom/gc-9-expected.txt:
* fast/dom/gc-9.html:
* fast/dom/node-childNodes-idempotence-expected.txt: Added.
* fast/dom/node-childNodes-idempotence.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105372 268f45cc-cd09-0410-ab3c-d52691b4dbfc