DevTools: make timeline record history experiment enabled by default

Bug: 726381
Change-Id: I76fb27d4eb50730ff85fdf39895859db4e343bc8
Reviewed-on: https://chromium-review.googlesource.com/811526
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523260}

third_party/WebKit/Source/devtools/front_end/main/Main.js

@@ -157,7 +157,7 @@ Main.Main = class {
 
     Runtime.experiments.setDefaultExperiments([
       'accessibilityInspection', 'colorContrastRatio', 'logManagement', 'performanceMonitor', 'persistence2',
-      'stepIntoAsync'
+      'stepIntoAsync', 'timelineKeepHistory'
     ]);
   }