[Lacros] Migrate ExtensionPlatformKeysService onto KeystoreService, Part 1

This CL is a part of a chain to migrate ExtensionPlatformKeysService
from using PlatformKeysService onto KeystoreService. This will
allow us to use it in both Ash-Chrome and Lacros-Chrome.

Implement KeystoreService::GenerateKey that forwards requests to
PlatformKeysService::GenerateKey and make
ExtensionPlatformKeysService use it.

Bug: 1152421, 1166105
Test: browsertest EnterprisePlatformKeys*
Change-Id: I91b0f872572e3dd016f5d2748f55f1b069545fde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2846509
Commit-Queue: Michael Ershov <miersh@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#879787}