Fix document mode migration.

Fix document mode migration.

There is an old tab_state file in the tabbed mode state directory. If
it is not deleted and the new tab state is written to the new
tab_state0 file, then when it comes time to run the migration code in
TabPersistentStore the old tab_state file will be given preference
and overwrite the new one with all the document mode tabs that were
just migrated. Thus we need to delete the old tab_state file if it
exists.

BUG=640625
Review-Url: https://codereview.chromium.org/2274943002
Cr-Commit-Position: refs/heads/master@{#414194}
(cherry picked from commit 9df9c7815b45e142c29162e02ddcdd8f411ec703)

Committed: https://chromium.googlesource.com/chromium/src/+/07d0458654375ff59c8b592ddd86c667587de678

[Peter Wen, 2016-08-29 13:22:46]

Description was changed from

==========
Fix document mode migration.

There is an old tab_state file in the tabbed mode state directory. If
it is not deleted and the new tab state is written to the new
tab_state0 file, then when it comes time to run the migration code in
TabPersistentStore the old tab_state file will be given preference
and overwrite the new one with all the document mode tabs that were
just migrated. Thus we need to delete the old tab_state file if it
exists.

BUG=640625

Review-Url: https://codereview.chromium.org/2274943002
Cr-Commit-Position: refs/heads/master@{#414194}
(cherry picked from commit 9df9c7815b45e142c29162e02ddcdd8f411ec703)
==========

to

==========
Fix document mode migration.

There is an old tab_state file in the tabbed mode state directory. If
it is not deleted and the new tab state is written to the new
tab_state0 file, then when it comes time to run the migration code in
TabPersistentStore the old tab_state file will be given preference
and overwrite the new one with all the document mode tabs that were
just migrated. Thus we need to delete the old tab_state file if it
exists.

BUG=640625

Review-Url: https://codereview.chromium.org/2274943002
Cr-Commit-Position: refs/heads/master@{#414194}
(cherry picked from commit 9df9c7815b45e142c29162e02ddcdd8f411ec703)

Committed:
https://chromium.googlesource.com/chromium/src/+/07d0458654375ff59c8b592ddd86...
==========

[Peter Wen, 2016-08-29 13:22:47]

Committed patchset #1 (id:1) manually as
07d0458654375ff59c8b592ddd86c667587de678.