Upon connection error during CDM initialization, after we rejected and
dropped the promise, MojoCdm could be destructed. We should not try to
close pending sessions in this case, and in fact we shouldn't have any
sessions open before the CDM is successfully initialized.

Added a unittest to cover the case where CDM creation failed due to
connection error.

BUG=652856,657069
TEST=Manually tested. Also added a new unittest.

Review-Url: https://codereview.chromium.org/2551333002
Cr-Commit-Position: refs/heads/master@{#436857}