I think the PositionExtrapolator is already in M51, and it is using elapsedRealtime.
What CL are you planing to merge?

https://codereview.chromium.org/1894013003

Thanks :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/27d06c547f130666676d1f016da58bd3e3b4c407

commit 27d06c547f130666676d1f016da58bd3e3b4c407
Author: avayvod <avayvod@chromium.org>
Date: Mon Apr 18 13:10:21 2016

[Cast, Android] Use elapsedRealtime vs uptimeMillis for end of playback UMA

BUG= 604345 

Review URL: https://codereview.chromium.org/1894013003

Cr-Commit-Position: refs/heads/master@{#387896}

[modify] https://crrev.com/27d06c547f130666676d1f016da58bd3e3b4c407/chrome/android/java/src/org/chromium/chrome/browser/media/remote/AbstractMediaRouteController.java

Your change meets the bar and is auto-approved for M51 (branch: 2704)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3c4c0dff08949d2c765f7a8d0ab3d9b361143008

commit 3c4c0dff08949d2c765f7a8d0ab3d9b361143008
Author: Anton Vayvod <avayvod@google.com>
Date: Mon Apr 18 13:39:00 2016

[Cast, Android] Use elapsedRealtime vs uptimeMillis for end of playback UMA

BUG= 604345 

Review URL: https://codereview.chromium.org/1894013003

Cr-Commit-Position: refs/heads/master@{#387896}
(cherry picked from commit 27d06c547f130666676d1f016da58bd3e3b4c407)

Review URL: https://codereview.chromium.org/1894123002 .

Cr-Commit-Position: refs/branch-heads/2704@{#90}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}

[modify] https://crrev.com/3c4c0dff08949d2c765f7a8d0ab3d9b361143008/chrome/android/java/src/org/chromium/chrome/browser/media/remote/AbstractMediaRouteController.java

Asking to merge to M50 - the risk is very low while it would save us 1.5 months waiting time for the metrics to get fixed in stable channel.

[Automated comment] Less than 2 weeks to go before stable on M50, manual review required.

Gotta land now if it's going to make 50.

Thanks, landing!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/67cc17978b217fc31d67352ca17fdfdd0175b0c0

commit 67cc17978b217fc31d67352ca17fdfdd0175b0c0
Author: Anton Vayvod <avayvod@google.com>
Date: Mon Apr 18 16:27:31 2016

[Cast, Android] Use elapsedRealtime vs uptimeMillis for end of playback UMA

BUG= 604345 

Review URL: https://codereview.chromium.org/1894013003

Cr-Commit-Position: refs/heads/master@{#387896}
(cherry picked from commit 27d06c547f130666676d1f016da58bd3e3b4c407)

Review URL: https://codereview.chromium.org/1895773003 .

Cr-Commit-Position: refs/branch-heads/2661@{#598}
Cr-Branched-From: ef6f6ae5e4c96622286b563658d5cd62a6cf1197-refs/heads/master@{#378081}

[modify] https://crrev.com/67cc17978b217fc31d67352ca17fdfdd0175b0c0/chrome/android/java/src/org/chromium/chrome/browser/media/remote/AbstractMediaRouteController.java