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[doc] Extend legacy IPC to Mojo conversion guide

This change adds information from the old wiki guide that was missing from the
current one.

Bug:917201

Change-Id: Ibfee4b8518e4e328dd3c4e1346d6ee729f4c0bfc
Reviewed-on: https://chromium-review.googlesource.com/c/1387457Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622133}