[Chromecast] Enable media autoplay without requiring user gesture

Bug: internal b/65850770
Test: cast video using Youtube app
Change-Id: Ic2093170c2fc2ecace6e8580a3dacabd504ca75e
Reviewed-on: https://chromium-review.googlesource.com/671133Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502915}

chromecast/browser/cast_browser_main_parts.cc

@@ -263,6 +263,9 @@ const DefaultCommandLineSwitch kDefaultSwitches[] = {
     // TODO(halliwell): Revert after fix for b/63101386.
     {switches::kDisallowNonExactResourceReuse, ""},
     {switches::kEnableMediaSuspend, ""},
+    // Enable autoplay without requiring any user gesture.
+    {switches::kAutoplayPolicy,
+     switches::autoplay::kNoUserGestureRequiredPolicy},
 };
 
 void AddDefaultCommandLineSwitches(base::CommandLine* command_line) {