Fix a timing bug in multibuffer.

Fix a timing bug in multibuffer.

When "cancel on defer is on", the multibuffer reader can sometimes get
delete before it is allowed to deliver a pending read callback. This basically
hangs the multibuffer data source as it waits for the read callback forever.

BUG=648395, 647867, 646434
Review-Url: https://codereview.chromium.org/2357773003
Cr-Commit-Position: refs/heads/master@{#419995}
(cherry picked from commit 3b2a5629da155db59d3449a6d640c107e91aa14d)

Committed: https://chromium.googlesource.com/chromium/src/+/b39d634c0efdd5912bfca4336ba9dc908e67fe10

[hubbe, 2016-09-22 22:40:45]

Description was changed from

==========
Fix a timing bug in multibuffer.

When "cancel on defer is on", the multibuffer reader can sometimes get
delete before it is allowed to deliver a pending read callback. This basically
hangs the multibuffer data source as it waits for the read callback forever.

BUG=648395, 647867, 646434

Review-Url: https://codereview.chromium.org/2357773003
Cr-Commit-Position: refs/heads/master@{#419995}
(cherry picked from commit 3b2a5629da155db59d3449a6d640c107e91aa14d)
==========

to

==========
Fix a timing bug in multibuffer.

When "cancel on defer is on", the multibuffer reader can sometimes get
delete before it is allowed to deliver a pending read callback. This basically
hangs the multibuffer data source as it waits for the read callback forever.

BUG=648395, 647867, 646434

Review-Url: https://codereview.chromium.org/2357773003
Cr-Commit-Position: refs/heads/master@{#419995}
(cherry picked from commit 3b2a5629da155db59d3449a6d640c107e91aa14d)

Committed:
https://chromium.googlesource.com/chromium/src/+/b39d634c0efdd5912bfca4336ba9...
==========

[hubbe, 2016-09-22 22:40:46]

Committed patchset #1 (id:1) manually as
b39d634c0efdd5912bfca4336ba9dc908e67fe10.