autocomplete: Make ContactProvider rank contacts by affinity

chrome/browser/chromeos/contacts/contact.proto

Index: chrome/browser/chromeos/contacts/contact.proto
diff --git a/chrome/browser/chromeos/contacts/contact.proto b/chrome/browser/chromeos/contacts/contact.proto
index 0cd6705241424adbd7852cdaff8e6ee66429752f..3d605e2283c083da1dbb0d3efbe14843434e2189 100644
--- a/chrome/browser/chromeos/contacts/contact.proto
+++ b/chrome/browser/chromeos/contacts/contact.proto
@@ -14,7 +14,7 @@ package contacts;
 // https://developers.google.com/gdata/docs/2.0/elements#gdContactKind
 // All strings are UTF-8 with Unicode byte order marks stripped out.
 message Contact {
-  // Next ID to use: 15
+  // Next ID to use: 16
 
   // Provider-assigned unique identifier.
   optional string contact_id = 1;
@@ -26,6 +26,10 @@ message Contact {
   // Has the contact been deleted recently within the upstream provider?
   optional bool deleted = 3 [default = false];
 
+  // Affinity between the user and this contact, in the range [0.0, 1.0].
+  // Unset if the affinity is unknown.
+  optional float affinity = 15;
+
   // Taken from https://developers.google.com/gdata/docs/2.0/elements#gdName.
   optional string full_name = 4;
   optional string given_name = 5;