[IndexedDB] Fixed accidental map insert of null backing store using []

This change fixes IndexedDBFactoryImpl by removing all "[]" access to
the |backing_store_map_|, which resulted in accidental insertions of
null backing stores in some cases.

This change also fixes a bunch of style nits where 'const auto&' was
being used for iterators instead of just 'auto'.

R=cmp@chromium.org

Bug: 942573, 944290
Change-Id: I673f626243dcad28794b8be1d823559e1d92759a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532909
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642755}