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This CL separates the part of LocalSharedObjectsContainer which is to be componentised into an interface, and keeps the LSOC itself an implementation detail of TabSpecificContentSettings (to be moved to the embedding client at a later point).

More design details here: https://docs.google.com/a/google.com/document/d/1rQ9PMCPL6wHh-QjaZWOBcaYXQgbCka7unSsciQ52qOs/edit#heading=h.hid3xtn12izy

BUG=384873

Review URL: https://codereview.chromium.org/588583002

Cr-Commit-Position: refs/heads/master@{#297802}