Add auto pairing to EasyUnlockPrivate interface.

Smart lock is needed at the end of Easy bootstrap to create proper
cryptohome keys for the user.

- Add EasyUnlockService::StartAutoPairing for Easy bootstrap code
  to start the auto pairing process and receive the result via a
  callback;
- Add onStartAutoPairing event to EasyUnlockPrivate. EasyUnlockService
  dispatches the event to trigger auto pairing in the app;
- Add setAutoPairingResult to EasyUnlockPrivate for the app to report
  the pairing result; The impl of the function invokes its counter part
  in EasyUnlockService to deliver the result to the callback provided
  in EasyUnlockService::StartAutoPairing call;

BUG=433133
TEST=EasyUnlockPrivateApiTest.AutoPairing

Review URL: https://codereview.chromium.org/910363002

Cr-Commit-Position: refs/heads/master@{#315776}
diff --git a/extensions/browser/extension_function_histogram_value.h b/extensions/browser/extension_function_histogram_value.h
index 70ac4061b..90c53ba 100644
--- a/extensions/browser/extension_function_histogram_value.h
+++ b/extensions/browser/extension_function_histogram_value.h
@@ -1017,6 +1017,7 @@
   NETWORKING_CONFIG_FINISHAUTHENTICATION,
   PLATFORMKEYSINTERNAL_SELECTCLIENTCERTIFICATES,
   CRYPTOTOKENPRIVATE_CANORIGINASSERTAPPID,
+  EASYUNLOCKPRIVATE_SETAUTOPAIRINGRESULT,
   // Last entry: Add new entries above and ensure to update
   // tools/metrics/histograms/histograms.xml.
   ENUM_BOUNDARY