DevTools: add tooltip for Preserve Log.

BUG=353994

Review URL: https://codereview.chromium.org/196383036

git-svn-id: svn://svn.chromium.org/blink/trunk@169557 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/devtools/front_end/NetworkPanel.js

@@ -650,6 +650,7 @@ WebInspector.NetworkLogView.prototype = {
         this._largerRequestsButton.addEventListener("click", this._toggleLargerRequests, this);
 
         this._preserveLogCheckbox = new WebInspector.StatusBarCheckbox(WebInspector.UIString("Preserve log"));
+        this._preserveLogCheckbox.element.title = WebInspector.UIString("Do not clear log on page reload / navigation.");
     },
 
     _loadEventFired: function(event)