[IndexedDB] Fixed transaction use-after-free vuln

      Bug: 725032
Change-Id: I689ded6c74d5563403587b149c3f3e02e807e4aa
Reviewed-on: https://chromium-review.googlesource.com/518483
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475952}