Android Media Notification: fix crash when stopping service with no running instance.

This fix has landed with https://codereview.chromium.org/1288803002, it
is therefore fixed in M46 onward. Given that the CL is large, it is not
cherry-picked but instead only the relevant line is merged to M45.

BUG=527668
R=avayvod@chromium.org

Review URL: https://codereview.chromium.org/1340773003 .

Cr-Commit-Position: refs/branch-heads/2454@{#469}
Cr-Branched-From: 12bfc3360892ec53cd00fc239a47e5298beb063b-refs/heads/master@{#338390}

• chrome/android/java/src/org/chromium/chrome/browser/media/ui/NotificationMediaPlaybackControls.java[diff]