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Introduces the concept of pending client initializations so that we
don't fail in odd ways if multiple initializations are in flight.

Also fixes some of the initialization PostTasks to ensure we're always
dereferencing our WeakPtrs on the same task runnerand ensuring that we
actually post our callbacks on the calling task runner.

Removes the WeakPtrFactory from ProtoLevelDBWrapper as well, which
involves a substantial refactoring to get DB init status back. A new
InitStatusCallback has been created so the 2 param Init and
InitWithDatabase calls give me an InitStatus. IsCorrupt was removed,
since setting the corruption state was the only thing that required
using a WeakPtr in the first place. This gives us the freedom to make
calls to the wrapper from any sequence regardless of what it was
created on, and not have the WeakPtrFactory cause problems when it's
destructed.

Bug: 912117,870813
Change-Id: Ic7931a543b4d3d09714184dfb335311130bc7667
Reviewed-on: https://chromium-review.googlesource.com/c/1364074
Commit-Queue: Troy Hildebrandt <thildebr@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616072}