Reland "[Lock screen media controls] Enable by default"

This reverts commit 2d943658.

Reason for revert: feature got all the approvals.

Original change's description:
> Revert "[Lock screen media controls] Enable by default"
> 
> This reverts commit 00d10067.
> 
> Reason for revert: feature not ready to launch in M78, reverting in trunk then merging.
> 
> Original change's description:
> > [Lock screen media controls] Enable by default
> > 
> > Enable the lock screen media controls by default
> > since these are shipping in M78.
> > 
> > BUG=965546
> > 
> > Change-Id: I159052753625fe4eb8bf3653bf2c7bdffe09615e
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773325
> > Reviewed-by: James Cook <jamescook@chromium.org>
> > Commit-Queue: Becca Hughes <beccahughes@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#690893}
> 
> TBR=jamescook@chromium.org,beccahughes@chromium.org
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: 965546
> Change-Id: I97211dd536f71ee8fed4b08b1124089343f797a9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1826043
> Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
> Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#700041}

TBR=jamescook@chromium.org,mlamouri@chromium.org,beccahughes@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 965546
Change-Id: I567d6fa05394e31df3d5e9a35f9c3bcc8af600ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867196Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707060}

ash/public/cpp/ash_features.cc

@@ -30,7 +30,7 @@ const base::Feature kLockScreenHideSensitiveNotificationsSupport{
     base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kLockScreenMediaControls{"LockScreenMediaControls",
-                                             base::FEATURE_DISABLED_BY_DEFAULT};
+                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kHideArcMediaNotifications{
     "HideArcMediaNotifications", base::FEATURE_ENABLED_BY_DEFAULT};