[Code Health] Clarify PaintWorkletPaintDispatcher behaviour

This CL adds some documentation for PaintWorkletPaintDispatcher, and
also changes up the |painter_map_| to allow direct lookup using the
worklet id.

Bug: None
Change-Id: I4f60ddce561b9fbc415f12f7f0bfcdd7a2c173d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625462
Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662634}
5 files changed