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This CL implements the sign() method from the platform_keys_internal
extension API. It adds a new Sign() method to the crosapi
KeystoreService that simply forwards the call to ash.

Change-Id: If696709159d2e8d97afd3133534f325f019344fb
Bug: 1164523, 1166105
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626086
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Michael Ershov <miersh@google.com>
Cr-Commit-Position: refs/heads/master@{#843385}