Migrate //crypto from base::Callback to base::OnceCallback

This updates //crypto to use base::OnceCallback instead of legacy
base::Callback. No intended functional change is included.

This reduces the number of 'base::Bind' in //crypto from 3 to 0,
as tracked at http://goo.gl/LUVhDj

Bug: 714018
Change-Id: I020993c154a0706a262543937d9ebe506ea01c25
Reviewed-on: https://chromium-review.googlesource.com/952507
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542030}

diff --git a/crypto/nss_util_internal.h b/crypto/nss_util_internal.h
index 080ac10..5fd40811 100644
--- a/crypto/nss_util_internal.h
+++ b/crypto/nss_util_internal.h

@@ -53,7 +53,7 @@
 // loaded and |callback| is non-null, the |callback| will be run once the slot
 // is loaded.
 CRYPTO_EXPORT ScopedPK11Slot GetSystemNSSKeySlot(
-    const base::Callback<void(ScopedPK11Slot)>& callback) WARN_UNUSED_RESULT;
+    base::OnceCallback<void(ScopedPK11Slot)> callback) WARN_UNUSED_RESULT;
 
 // Sets the test system slot to |slot|, which means that |slot| will be exposed
 // through |GetSystemNSSKeySlot| and |IsTPMTokenReady| will return true.
@@ -102,7 +102,7 @@
 // is loaded.
 CRYPTO_EXPORT ScopedPK11Slot GetPrivateSlotForChromeOSUser(
     const std::string& username_hash,
-    const base::Callback<void(ScopedPK11Slot)>& callback) WARN_UNUSED_RESULT;
+    base::OnceCallback<void(ScopedPK11Slot)> callback) WARN_UNUSED_RESULT;
 
 // Closes the NSS DB for |username_hash| that was previously opened by the
 // *Initialize*ForChromeOSUser functions.