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

Bug: 955171
Change-Id: I764b3209cd35ec5593088c4b81e32167f3417b8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810503
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697593}