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This gathers the code relating to SourceId into a new file, and adds
a conversion method from other ID types.  External ID types aren't
converted when recieving IDs from other processes.

Change-Id: I3c65406457666f9d099d654a75db7e13a53590d7
Reviewed-on: https://chromium-review.googlesource.com/596725
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Oystein Eftevaag <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491602}