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The parent_offset currently stored at each transform tree node sometimes
depends on the transform between some other node (which happens to
correspond to node's owning layer's parent's transform node) and the node's
parent. When this transform changes and the transform tree isn't rebuilt,
the parent_offset becomes stale. This CL replaces parent_offset with
source_offset, which is the portion of parent_offset that doesn't depend on
a transform between tree nodes, and adds a source_node_id to each node
so that the transform between nodes (when needed) can be recomputed
when the transform tree is updated.

BUG=480589

Review URL: https://codereview.chromium.org/1106013002

Cr-Commit-Position: refs/heads/master@{#326894}