sync: Add pre-commit update avoidance mode + flag

sync/protocol/experiments_specifics.proto

Index: sync/protocol/experiments_specifics.proto
diff --git a/sync/protocol/experiments_specifics.proto b/sync/protocol/experiments_specifics.proto
index f834cb91c039297b50d29c9c3d136e78b2b582ca..1e8732fd028ea2fc80dd8132bc9ef313e7fbbb50 100644
--- a/sync/protocol/experiments_specifics.proto
+++ b/sync/protocol/experiments_specifics.proto
@@ -34,6 +34,11 @@ message FaviconSyncFlags {
   optional int32 favicon_sync_limit = 2 [default = 200];
 }
 
+// Flags for enabling the experimental no-precommit GU feature.
+message PreCommitUpdateAvoidanceFlags {
+  optional bool enabled = 1;
+}
+
 // Contains one flag or set of related flags.  Each node of the experiments type
 // will have a unique_client_tag identifying which flags it contains.  By
 // convention, the tag name should match the sub-message name.
@@ -42,4 +47,5 @@ message ExperimentsSpecifics {
   optional HistoryDeleteDirectives history_delete_directives = 2;
   optional AutofillCullingFlags autofill_culling = 3;
   optional FaviconSyncFlags favicon_sync = 4;
+  optional PreCommitUpdateAvoidanceFlags pre_commit_update_avoidance = 5;
 }