IndexedDB: Protect against use-after-free in ChainedBlobWriter.

IndexedDB: Protect against use-after-free in ChainedBlobWriter.

This is a speculative fix for a heap user-after-free bug. Was unable
to verify using a Windows SyzyASan build. The theory is that if Abort()
was called before ChainedBlobWriterImpl::WriteNextFile() could set
waiting_for_callback_ then the ReportWriteCompletion() would never know
that it was aborted and attempt to use it's dangling raw pointer to a
deleted IndexedDBBackingStore instance.

Also in this change is the elimination of the redundant aborted_
member variable.

BUG=472614
Committed: https://crrev.com/29777a8ee0f45b8160ec004e74013d5b62b6828a
Cr-Commit-Position: refs/heads/master@{#326597}

[cmumford, 2015-04-03 17:56:59]

Still trying to repro on Windows, but wanted to throw up this CL for
consideration.

[cmumford, 2015-04-21 21:49:04]

cmumford@chromium.org changed reviewers:
+ jsbell@chromium.org

[cmumford, 2015-04-21 21:49:04]

jsbell: I tried again today, but cannot repro the SyzyASan failure for the
associated bug. It's possible (but unlikely) that this change fixes this issue,
but it does have one DCHECK which should shed some light on how this object is
getting deleted with an outstanding callback.

[jsbell, 2015-04-22 23:49:47]

lgtm

[cmumford, 2015-04-23 17:48:56]

The CQ bit was checked by cmumford@chromium.org

[commit-bot: I haz the power, 2015-04-23 17:50:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1060613002/10002

[commit-bot: I haz the power, 2015-04-23 18:56:22]

Committed patchset #2 (id:10002)

[commit-bot: I haz the power, 2015-04-23 18:58:11]

Patchset 2 (id:??) landed as
https://crrev.com/29777a8ee0f45b8160ec004e74013d5b62b6828a
Cr-Commit-Position: refs/heads/master@{#326597}