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This change is the first one in a series to start removing blob-specific
code from the IndexedDBBackingStore class into its own file. This will
eventually hold the blob storage integration separately with its own
tests, etc.

Work planning doc:
https://docs.google.com/document/d/18suNOOzuEJbqgRJF0MB2VqdTyYqS4cvI2PGaCpyPXSw/edit?pli=1#heading=h.ilyrswbua4hv

Bug: 1024966
Change-Id: I3a468f79720344437973b175b098ad60389079fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918199Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Auto-Submit: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719423}