Reland PaintChunk::id

The id will be used in rasterization paint invalidation to match old
and new paint chunks. It can be also used in incremental layerization.

Original review url: https://codereview.chromium.org/2116693002

BUG=510908,629946

Review-Url: https://codereview.chromium.org/2116693002
Review-Url: https://codereview.chromium.org/2168513005
Cr-Original-Commit-Position: refs/heads/master@{#406461}
Cr-Commit-Position: refs/heads/master@{#406793}
20 files changed