webauthn: use cached JNI classes.

The JNI wrapper code already caches the class objects. Use them in
DCHECKs rather than doing a lookup every time. Also means that typoing
the class name is now a compile-time failure.

BUG=1002262

Change-Id: Ifc86bdc859d6529d0aeaa54f99c63ad199a8b8a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453919
Auto-Submit: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#814599}