IndexedDB: Address performance regression with blob journals

A fix at r313998 corrected blob accounting during transaction commits.
To avoid stomping on overlapping commits, it required reading journals
from the backing store, which regressed performance tests. Scope the
code to only modify the journals if the transaction touches blobs.

R=cmumford@chromium.org
BUG=454622

Review URL: https://codereview.chromium.org/902503003

Cr-Commit-Position: refs/heads/master@{#315592}
2 files changed