Fix document mode migration again

Fix document mode migration again

The document mode migration code was changed in
4804efe6387351c7802a2e9f34afe84e7ccbe7fe
to skip document mode migration if a shared preference was not set.
On older versions of Chrome, that shared preference wasn't set
even though the user was in document mode. Migration was skipped and
all tabs were lost.

Revert to the old logic of using another shared preference to help
determine if the user is in document mode.

BUG=646146
TBR=dfalcantara@chromium.org

[gone, 2016-09-14 23:04:56]

lgtm

[Theresa, 2016-09-21 16:56:26]

Description was changed from

==========
Fix document mode migration again

The document mode migration code was changed in
4804efe6387351c7802a2e9f34afe84e7ccbe7fe
to skip document mode migration if a shared preference was not set.
On older versions of Chrome, that shared preference wasn't set
even though the user was in document mode. Migration was skipped and
all tabs were lost.

Revert to the old logic of using another shared preference to help
determine if the user is in document mode.

BUG=646146
TBR=dfalcantara@chromium.org
==========

to

==========
Fix document mode migration again

The document mode migration code was changed in
4804efe6387351c7802a2e9f34afe84e7ccbe7fe
to skip document mode migration if a shared preference was not set.
On older versions of Chrome, that shared preference wasn't set
even though the user was in document mode. Migration was skipped and
all tabs were lost.

Revert to the old logic of using another shared preference to help
determine if the user is in document mode.

BUG=646146
TBR=dfalcantara@chromium.org
==========