Enable accelerated plugins

This allows the use of Stage Video in Pepper Flash in non-fullscreen mode, and
works around a Youtube bug wrt the transition fullscreen <-> normal.

Change-Id: Ibb86500bdd2dec4a3cfd713162e5414e8c572edd

BUG=none
TEST=Youtube.

Review URL: http://codereview.chromium.org/6626061
diff --git a/session_manager_setup.sh b/session_manager_setup.sh
index 9199f45..00d6ef2 100755
--- a/session_manager_setup.sh
+++ b/session_manager_setup.sh
@@ -287,6 +287,7 @@
             --device-management-url="$DMSERVER" \
             --disable-domui-menu \
             --disable-seccomp-sandbox \
+            --enable-accelerated-plugins \
             --enable-gview \
             --enable-logging \
             --enable-login-images \