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If an incorrect type is provided to the call, the generated IDL code will
reject with a TypeError. This patch removes code in the service
implementation which was checking whether the type was invalid and returning
a NotSupportedError. The only way this code could be hit would be if there
was a programming error, so it should be a DCHECK.

BUG=672113

Review-Url: https://codereview.chromium.org/2570463003
Cr-Commit-Position: refs/heads/master@{#437897}