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As part of cleanup from CL https://crrev.com/c/1626660, this CL
moves creation of CursorImpl into IndexedDBDispatcherHost by
refactoring AddCursorBinding into CreateCursorBinding.

As a result, the OpenCursor calling path no longer needs to pass
the origin and sequenced task runner into the OpenCursorOperation
task and ownership of CursorImpl and the mojo binding steps is
made simpler.

Bug: 988868
Change-Id: Iab0be1d24d98fc6b0088d01c67c1b778c5c13318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725318Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682428}