[Sync] Add ProfileSyncService::GetModelTypeStoreFactory.

[Sync] Add ProfileSyncService::GetModelTypeStoreFactory.

This allows other model types besides DEVICE_INFO to create
ModelTypeStores that share the same LevelDB backend.

BUG=685451
Review-Url: https://codereview.chromium.org/2658743002
Cr-Commit-Position: refs/heads/master@{#446372}
(cherry picked from commit 9f1ddc6deb94f709ca8a8635d270749560295a72)

Review-Url: https://codereview.chromium.org/2669283003 .
Cr-Commit-Position: refs/branch-heads/2987@{#283}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/2ebeaa02e3d830ebd339be4134252682fe600330

[pavely, 2017-02-02 22:04:22]

Description was changed from

==========
[Sync] Add ProfileSyncService::GetModelTypeStoreFactory.

This allows other model types besides DEVICE_INFO to create
ModelTypeStores that share the same LevelDB backend.

BUG=685451

Review-Url: https://codereview.chromium.org/2658743002
Cr-Commit-Position: refs/heads/master@{#446372}
(cherry picked from commit 9f1ddc6deb94f709ca8a8635d270749560295a72)
==========

to

==========
[Sync] Add ProfileSyncService::GetModelTypeStoreFactory.

This allows other model types besides DEVICE_INFO to create
ModelTypeStores that share the same LevelDB backend.

BUG=685451

Review-Url: https://codereview.chromium.org/2658743002
Cr-Commit-Position: refs/heads/master@{#446372}
(cherry picked from commit 9f1ddc6deb94f709ca8a8635d270749560295a72)

Review-Url: https://codereview.chromium.org/2669283003 .
Cr-Commit-Position: refs/branch-heads/2987@{#283}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/2ebeaa02e3d830ebd339be413425...
==========

[pavely, 2017-02-02 22:04:24]

Committed patchset #1 (id:1) manually as
2ebeaa02e3d830ebd339be4134252682fe600330.