Remove isSyncDatabase in WebDatabaseObserver [3/3]

Final clean-up for methods with isSyncDatabase parameter.

[1] https://codereview.chromium.org/660873004/
[2] https://codereview.chromium.org/666333002/
[3] THIS PATCH

BUG=428254

Review URL: https://codereview.chromium.org/669063002

git-svn-id: svn://svn.chromium.org/blink/trunk@184779 bbb929c8-8fbe-4397-9dbb-9b2b20218538