login: Re-enable prefixed EME by default in Chrome OS.

BUG=chromium:554652
TEST=none

Change-Id: I86615bc0c6ec69ceb1f8c99103bcec93cac40a87
Reviewed-on: https://chromium-review.googlesource.com/312310
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
(cherry picked from commit d4629233bb3ebaba57d8e8de5107fc748a00bc90)
Reviewed-on: https://chromium-review.googlesource.com/312974
Reviewed-by: Zelidrag Hornung <zelidrag@chromium.org>
Commit-Queue: Zelidrag Hornung <zelidrag@chromium.org>
Trybot-Ready: Zelidrag Hornung <zelidrag@chromium.org>
Tested-by: Zelidrag Hornung <zelidrag@chromium.org>
diff --git a/login_manager/chrome_setup.cc b/login_manager/chrome_setup.cc
index 029bece..fd69d95 100644
--- a/login_manager/chrome_setup.cc
+++ b/login_manager/chrome_setup.cc
@@ -245,6 +245,9 @@
   // at crbug.com/309806.
   if (builder->UseFlagIsSet("moblab"))
     builder->AddArg("--disable-demo-mode");
+
+  // Re-enable prefixed EME by default in Chrome OS.
+  builder->AddArg("--enable-prefixed-encrypted-media");
 }
 
 // Adds enterprise-related flags to the command line.