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Due to an issue in the CDM, when a session to be loaded is not
found, the CDM calls PromiseResolve() instead of the expected
PromiseResolveWithSession(""). This change detects this situation
and handles it properly.

BUG=408337
TEST=existing EME tests pass

Review URL: https://codereview.chromium.org/518523002

Cr-Commit-Position: refs/heads/master@{#292497}