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IndexedDB: Tighten up IndexedDBBackingStore thread safety checks.

This CL updates IndexedDBBackingStore and its inner classes.

* Each class is documented as not thread-safe. The obsolete thread
  safety comment on IndexedDBBackingstore is removed.

* Each class' data members are either marked const or
  GUARDED_BY_CONTEXT with a SequenceChecker.

* DISALLOW_COPY_AND_ASSIGN is replaced with deleted copy constructor and
  assignment.

* An unused data member was removed.

* Data members initialized with constants in constructors are instead
  initialized when declared.

* Invariant-enforcing DCHECKs are consistently placed at the beginning
  of methods.

This CL introduces no functional changes.

Change-Id: I8a38d0f2225ff4b5334fd37891a8216b8bb72930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2588292Reviewed-by: Chris Mumford <cmumford@google.com>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836952}