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Adds a SharedProtoDatabase that owns a LevelDB instance. The shared
database provides a way to get SharedProtoDatabaseClients to access
the shared DB with a unique namespace/prefix. The SharedProtoDatabase
instance is provided through the KeyedService ProtoDatabaseProvider
so the shared database is tied to the appropriate profile directory.

Bug: 870813
Change-Id: I433518c3fee4d37abcaa0c7efa94e7912da521ec
Reviewed-on: https://chromium-review.googlesource.com/c/1332009Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Troy Hildebrandt <thildebr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612000}