Change other usages of .size() to .empty() when applicable.

chrome/browser/sync/glue/foreign_session_tracker.cc

Index: chrome/browser/sync/glue/foreign_session_tracker.cc
diff --git a/chrome/browser/sync/glue/foreign_session_tracker.cc b/chrome/browser/sync/glue/foreign_session_tracker.cc
index 6db5c9cb385415521763ce2ac2400646a65706ed..e9bc9c68faef7445e350ff9448330db01a8e33d0 100644
--- a/chrome/browser/sync/glue/foreign_session_tracker.cc
+++ b/chrome/browser/sync/glue/foreign_session_tracker.cc
@@ -23,17 +23,14 @@ bool ForeignSessionTracker::LookupAllForeignSessions(
     foreign_session_map_.begin(); i != foreign_session_map_.end(); ++i) {
     // Only include sessions with open tabs.
     ForeignSession* foreign_session = i->second;
-    if (foreign_session->windows.size() > 0 &&
+    if (!foreign_session->windows.empty() &&
         !SessionModelAssociator::SessionWindowHasNoTabsToSync(
             *foreign_session->windows[0])) {
       sessions->push_back(foreign_session);
     }
   }
 
-  if (sessions->size() > 0)
-    return true;
-  else
-    return false;
+  return !sessions->empty();
 }
 
 bool ForeignSessionTracker::LookupSessionWindows(