Seperate modifications of property tree element ids from layer element id map.

To avoid overlap, we should separate the methods that deal with ensuring we maintain the map from element id to layer and the list of element ids in the property trees.

Bug: 934511
Change-Id: I1e1d4ca3d11ae9becec208748701358b31facba0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506592
Commit-Queue: Robert Flack <flackr@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646064}