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This CL converts KeySystemSupport{Ptr, Request} in content to the
new Mojo type.

Bug: 955171
Change-Id: I84b54952523842d408f13515eeae0649c73895e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1882200Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/master@{#710133}