Document that HistoryBackend is sequence-affine, not thread-affine.

Document that HistoryBackend is sequence-affine, not thread-affine.

HistoryBackend methods must not be called concurrently and external
synchronization is required between calls on different thread.
However, the methods do not have to be called from a single thread.
This CL documents this and adds DCHECKs to verify that
HistoryBackend methods are called in sequence.

BUG=675800, 675631
Committed: https://crrev.com/0b0b5aab6e2474e0e758ca3777b07321e59a7463
Cr-Commit-Position: refs/heads/master@{#440408}

[fdoray, 2016-12-21 15:03:38]

fdoray@chromium.org changed reviewers:
+ sdefresne@chromium.org

[fdoray, 2016-12-21 15:03:38]

PTAL

[sdefresne, 2016-12-22 13:10:36]

lgtm

[fdoray, 2016-12-22 13:20:47]

The CQ bit was checked by fdoray@chromium.org

[commit-bot: I haz the power, 2016-12-22 13:21:12]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-22 14:09:15]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1482412847211260,
"parent_rev": "3cd71715060ce6c33db03a38243c4629fbf87f7d", "commit_rev":
"1f1157fe2d26bf141c480ea476f9370d0a9e8951"}

[commit-bot: I haz the power, 2016-12-22 14:09:45]

Description was changed from

==========
Document that HistoryBackend is sequence-affine, not thread-affine.

HistoryBackend methods must not be called concurrently and external
synchronization is required between calls on different thread.
However, the methods do not have to be called from a single thread.
This CL documents this and adds DCHECKs to verify that
HistoryBackend methods are called in sequence.

BUG=675800, 675631
==========

to

==========
Document that HistoryBackend is sequence-affine, not thread-affine.

HistoryBackend methods must not be called concurrently and external
synchronization is required between calls on different thread.
However, the methods do not have to be called from a single thread.
This CL documents this and adds DCHECKs to verify that
HistoryBackend methods are called in sequence.

BUG=675800, 675631

Review-Url: https://codereview.chromium.org/2591123004
==========

[commit-bot: I haz the power, 2016-12-22 14:09:46]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-12-22 14:13:20]

Description was changed from

==========
Document that HistoryBackend is sequence-affine, not thread-affine.

HistoryBackend methods must not be called concurrently and external
synchronization is required between calls on different thread.
However, the methods do not have to be called from a single thread.
This CL documents this and adds DCHECKs to verify that
HistoryBackend methods are called in sequence.

BUG=675800, 675631

Review-Url: https://codereview.chromium.org/2591123004
==========

to

==========
Document that HistoryBackend is sequence-affine, not thread-affine.

HistoryBackend methods must not be called concurrently and external
synchronization is required between calls on different thread.
However, the methods do not have to be called from a single thread.
This CL documents this and adds DCHECKs to verify that
HistoryBackend methods are called in sequence.

BUG=675800, 675631

Committed: https://crrev.com/0b0b5aab6e2474e0e758ca3777b07321e59a7463
Cr-Commit-Position: refs/heads/master@{#440408}
==========

[commit-bot: I haz the power, 2016-12-22 14:13:21]

Patchset 2 (id:??) landed as
https://crrev.com/0b0b5aab6e2474e0e758ca3777b07321e59a7463
Cr-Commit-Position: refs/heads/master@{#440408}

[Peng, 2016-12-22 16:54:10]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/2599803002/ by penghuang@chromium.org.

The reason for reverting is:
https://uberchromegw.corp.google.com/i/chromium.chromiumos/builders/Linux%20C...

The CL causes the browser_tests failure on Linux ChromiumOS Tests(dbg) .