Trust Tokens: Clarify a comment in ExpiryInspectingRecordExpiryDelegate

ExpiryInspectingRecordExpiryDelegate uses Trust Tokens signed redemption
records' associated token issuance verification keys in order to decide
whether a given signed redemption record has expired. This is a little
subtle, because signed redemption records *also* have associated public-
key signatures, verified by "signed redemption record verification keys,"
which are distinct from SRRs' associated token issuance verification
keys.

For the most part, the comments in
expiry_inspecting_record_expiry_delegate.{h,cc} go out of their way to
make the distinction explicit, but there was a comment that mentioned
the SRR's "verification key", rather than its associated token issuance
verification key.

This patch updates the comment in question to be clearer.

Tbr: svaldez@chromium.org
Change-Id: I87f45aa411c9c4462bf7fd4ce00a92ca41056daa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429304Reviewed-by: Steven Valdez <svaldez@chromium.org>
Reviewed-by: David Van Cleve <davidvc@chromium.org>
Commit-Queue: Steven Valdez <svaldez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810345}

services/network/trust_tokens/expiry_inspecting_record_expiry_delegate.cc

@@ -93,9 +93,10 @@ bool ExpiryInspectingRecordExpiryDelegate::IsRecordExpired(
   // changed (due to data corruption) or the commitments changed (due to an
   // overwrite by the key commitments' producer, or due to data corruption).
   //
-  // In both cases, the SRR's verification key doesn't correspond to the current
-  // key commitments we possess for the issuer, because we don't have any
-  // key commitments whatsoever for the issuer: mark the record as expired.
+  // In both cases, the SRR's associated token-issuance verification key isn't
+  // present in the current key commitments we possess for the issuer, because
+  // we don't have any key commitments whatsoever for the issuer: mark the
+  // record as expired.
   if (!key_commitments)
     return true;