Remove isSyncDatabase in WebDatabaseObserver [2/3]

Removed codes related to isSyncDatabase and switched to the new methods
that were introduced in the previous patch.

[1] https://codereview.chromium.org/660873004/
[2] THIS PATCH
[3] https://codereview.chromium.org/669063002/

BUG=428254

Review URL: https://codereview.chromium.org/666333002

Cr-Commit-Position: refs/heads/master@{#302301}
2 files changed