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The parameter isn't needed because the other object already carries the
key system information. This CL extends the interface so the Blink
object can get the key system from the implementation through it.

This is needed to reduce the boilerplate in the Media Capabilities
Encrypted Media implementation.

Bug: 907909
Change-Id: If59a6fc0ef47292696b5538a70b7fd22fbf024ca
Reviewed-on: https://chromium-review.googlesource.com/c/1348731
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611407}