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Current WebContentDecryptionModuleImpl and CdmSessionAdapter take a
blink::WebContentDecryptionModuleResult during CDM creation. There are a
few issues with this:
1. Bad encapsulation, e.g. CompleteWithSession() is exposed to these
   two classes.
2. Hard to test: There's no easy way to create one |result| for testing.
   See Bug.
3. Readability: Working with the result directly ends up with more code,
   e.g. kWebContentDecryptionModuleExceptionNotSupportedError.

Bug: 1018832
Test: No functionality change
Change-Id: I9b1ec2662873d9da6797133355b1a3261145feac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884277Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710617}