chrome.gcdPrivate allows app to choose pairing method.
Added startPairing with pairing method parameter.
Changed some naming in gcd_private.idl
Changed PrivetV3Session from delegates to Callback. This makes code smaller and more readable.

Review URL: https://codereview.chromium.org/695253002

Cr-Commit-Position: refs/heads/master@{#302565}
diff --git a/extensions/browser/extension_function_histogram_value.h b/extensions/browser/extension_function_histogram_value.h
index f1f2a63..476f9779 100644
--- a/extensions/browser/extension_function_histogram_value.h
+++ b/extensions/browser/extension_function_histogram_value.h
@@ -973,6 +973,7 @@
   HOTWORDPRIVATE_STARTTRAINING,
   HOTWORDPRIVATE_FINALIZESPEAKERMODEL,
   HOTWORDPRIVATE_STOPTRAINING,
+  GCDPRIVATE_STARTPAIRING,
   // Last entry: Add new entries above and ensure to update
   // tools/metrics/histograms/histograms.xml.
   ENUM_BOUNDARY