Reduce size of HTMLCollection / LiveNodeList / ChildNodeList by 1 word

Reduce size of HTMLCollection / LiveNodeList / ChildNodeList by 1 word by
updating the bitfield in CollectionIndexCache so that m_cachedNodeIndex and
m_isLengthCacheValid share 32 bits. Otherwise, the compiler was adding padding
bits after m_isLengthCacheValid to get proper memory alignment.

On my 64 bit Linux machine with gcc 4.8, I see the following size changes:
* HTMLCollection: 72 bytes -> 64 bytes
* LiveNodeList: 72 bytes -> 64 bytes
* ChildNodeList: 56 bytes -> 48 bytes

I ran PerformanceTests/Bindings/node-list-access.html performance test and
could not see any performance impact with this CL.

R=esprehn@chromium.org
BUG=

Review URL: https://codereview.chromium.org/326073003

git-svn-id: svn://svn.chromium.org/blink/trunk@176603 bbb929c8-8fbe-4397-9dbb-9b2b20218538