sync: Add pre-commit update avoidance mode + flag

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 02128fbd1797dfbfbf79df67f4ceb792eb6cc19d..4aa10ea701288e34f839d2cab085b5d1e9b29000 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1351,6 +1351,10 @@ const char kSyncEnableDeferredStartup[]     = "sync-enable-deferred-startup";
 // ClientLogin token.
 const char kSyncDisableOAuth2Token[]         = "sync-disable-oauth2-token";
 
+// Enables feature to avoid unnecessary GetUpdate requests.
+const char kSyncEnableGetUpdateAvoidance[]   =
+    "sync-enable-get-update-avoidance";
+
 // Enables directory support for sync filesystem
 const char kSyncfsEnableDirectoryOperation[] =
     "enable-syncfs-directory-operation";