Autoplay: add document user activation flag in chrome://flags

This is plumbing the flag to Blink. The feature is not fully implemented
but can be tested. The missing parts are:
- Media Engagement Index;
- Iframe delecation;
- Sticky bit across navigations.

BUG=715049

Review-Url: https://codereview.chromium.org/2921273002
Cr-Commit-Position: refs/heads/master@{#477238}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index a3f2521..1e03569 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -757,6 +757,9 @@
      switches::kAutoplayPolicy,
      switches::autoplay::kUserGestureRequiredForCrossOriginPolicy},
 #endif
+    {flag_descriptions::kAutoplayPolicyDocumentUserActivation,
+     switches::kAutoplayPolicy,
+     switches::autoplay::kDocumentUserActivationRequiredPolicy},
 };
 
 const FeatureEntry::Choice kForceEffectiveConnectionTypeChoices[] = {