[IndexedDB] Fixed transaction use-after-free vuln

TBR=dmurph@chromium.org

(cherry picked from commit 11601c08e92732d2883af2057c41c17cba890844)

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-Original-Commit-Position: refs/heads/master@{#475952}
Reviewed-on: https://chromium-review.googlesource.com/521762
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/branch-heads/3112@{#97}
Cr-Branched-From: b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
2 files changed