Make PaintImages a class and implement a Clone() method.

Currently we use Vector<Persistent<StyleImage>> to represent a list
of paint images. When we generate StyleRareNonInheritedData, we need
this type to have a Clone method because it is stored inside a
std::unique_ptr. This patch makes this type its own class so that
it can be extended with a Clone method.

BUG=628043

Review-Url: https://codereview.chromium.org/2911343002
Cr-Commit-Position: refs/heads/master@{#476580}
5 files changed