Figured out root cause, tab_state file is not deleted, so legacy migration task runs (if it has not run before) and replaces tab_state0 with tab_state (which only contained the new tab), or the new multi-instance migration runs which overwrites tab_state0 with tab_state.

Well, I figured this would happen with multiple migration pathways happening.  The document mode migration tests didn't account for that.

Adding this to tests and fix on the way. Will have to be merged into M53 as the other migration code for tab_state0 is in M53.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9df9c7815b45e142c29162e02ddcdd8f411ec703

commit 9df9c7815b45e142c29162e02ddcdd8f411ec703
Author: wnwen <wnwen@chromium.org>
Date: Wed Aug 24 23:21:13 2016

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}

[modify] https://crrev.com/9df9c7815b45e142c29162e02ddcdd8f411ec703/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java
[modify] https://crrev.com/9df9c7815b45e142c29162e02ddcdd8f411ec703/chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassinTest.java

Merging fix in #4 for users updating to M53 from <M51.

[Automated comment] Less than 2 weeks to go before stable on M53, manual review required.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0b8739a23d597e4602e4de5a794908d9820e0bcc

commit 0b8739a23d597e4602e4de5a794908d9820e0bcc
Author: wnwen <wnwen@chromium.org>
Date: Thu Aug 25 21:14:02 2016

Clarify full and prefix variables.

BUG= 640625 

Review-Url: https://codereview.chromium.org/2275113004
Cr-Commit-Position: refs/heads/master@{#414539}

[modify] https://crrev.com/0b8739a23d597e4602e4de5a794908d9820e0bcc/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java
[modify] https://crrev.com/0b8739a23d597e4602e4de5a794908d9820e0bcc/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabbedModeTabPersistencePolicy.java
[modify] https://crrev.com/0b8739a23d597e4602e4de5a794908d9820e0bcc/chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassinTest.java
[modify] https://crrev.com/0b8739a23d597e4602e4de5a794908d9820e0bcc/chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/MultiInstanceMigrationTest.java
[modify] https://crrev.com/0b8739a23d597e4602e4de5a794908d9820e0bcc/chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/RestoreMigrateTest.java

Merge of c#4 approved for M53 branch 2785, please re-apply label if you need c#8 as well.

This issue has been approved for a merge. Please merge the fix to any appropriate branches as soon as possible!

If all merges have been completed, please remove any remaining Merge-Approved labels from this issue.

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/07d0458654375ff59c8b592ddd86c667587de678

commit 07d0458654375ff59c8b592ddd86c667587de678
Author: Peter Wen <wnwen@google.com>
Date: Mon Aug 29 13:17:59 2016

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)

Review URL: https://codereview.chromium.org/2290633002 .

Cr-Commit-Position: refs/branch-heads/2785@{#777}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/07d0458654375ff59c8b592ddd86c667587de678/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassin.java
[modify] https://crrev.com/07d0458654375ff59c8b592ddd86c667587de678/chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/DocumentModeAssassinTest.java