Add field to monitor last visited time for each search engine

components/sync/protocol/search_engine_specifics.proto

Index: components/sync/protocol/search_engine_specifics.proto
diff --git a/components/sync/protocol/search_engine_specifics.proto b/components/sync/protocol/search_engine_specifics.proto
index 03a3e07f8066bfa93068c68453600909a4cab7db..790b87b5a019964544c5965c3354e29899a50769 100644
--- a/components/sync/protocol/search_engine_specifics.proto
+++ b/components/sync/protocol/search_engine_specifics.proto
@@ -78,4 +78,9 @@ message SearchEngineSpecifics {
 
   // The parameterized URL for a search provider specified new tab page.
   optional string new_tab_url = 26;
+
+  // The date this search engine entry was last visited. A UTC timestamp with
+  // units

[Peter Kasting, 2016/11/21 03:35:08]

Nit: Broken line wrapping

This usually happens if git cl format reformats a comment that was too long.  If
you haven't already done so, consider modifying your editor of choice so the
80-column mark is clearly visible and you'll be unlikely to write comments that
exceed it.

[ltian, 2016/11/28 22:08:02]

I know Java's maximum column size is 100 and C/C++ one is 80. But do you have
any idea what are the maximum column sizes for the other types of files? I
cannot find reference of them. 

[Peter Kasting, 2016/11/28 22:20:24]

I don't know.  I would assume 80 columns in the absence of other info.

+  // in microseconds.
+  optional int64 last_visited = 27;
 }