Remove additionalOutlineSize in ShadowList: it is always 0.

Review URL: https://codereview.chromium.org/479563002

git-svn-id: svn://svn.chromium.org/blink/trunk@180370 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Rename CollectionIndexCache API to not use 'Element'

Rename CollectionIndexCache API to not use 'Element':
- traverseToFirstElement() -> traverseToFirst()
- traverseToLastElement() -> traverseToLast()

Using Element is confusing because some CollectionIndexCache users like
ChildNodeList deal with pure Nodes. The Node type is a template parameter
of the CollectionIndexCache class so the API should be seem to be Element
specific.

R=rob.buis@samsung.com

Review URL: https://codereview.chromium.org/469913002

git-svn-id: svn://svn.chromium.org/blink/trunk@180371 bbb929c8-8fbe-4397-9dbb-9b2b20218538