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While here, this CL also marks some IndexedDBContextImpl data members
const. No functional changes are introduced.

Change-Id: I45d4b887d6a9fcf6a3719cc32e745837594dd4e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2609904Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840445}