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This is the first component of LevelDB Scopes. The lock manager is a
basic leveled lock system with shared and exclusive locks. This change
contains an implementation which requires disjoint lock ranges, which
is what the IndexedDB system will be using.

This also includes a README with the implementation plan. This will be
updated as more code is submitted.

Bug: 862456
Change-Id: Ib4300ae8c59035c06772625323b3de0cc9ef3f6a
Reviewed-on: https://chromium-review.googlesource.com/1134407
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581385}