Edit for "additional notes" above
Chrome Browser Beta 52.0.2743.49 has --enable-unified-media-pipeline flag set ON by default (if more precise, it is now called --disable-unified-media-pipeline and is OFF by default). Thus things work on Chrome Browser if unified media pipeline for Android is on. Just wanted to add this as observed hint where the issue maybe lies. Anyways, WebView cannot render looping video. And no way for USER to set flags (thus forget about https://android.googlesource.com/platform/frameworks/webview/+/android-cts-5.1_r8/chromium/java/com/android/webview/chromium/WebViewChromiumFactoryProvider.java#76)

passing to Tima

Spitzer has been enabled for WebView by https://codereview.chromium.org/2057743002/, commit
https://chromium.googlesource.com/chromium/src/+/e0e5c818737bb30a4526245db6ef32d9c6a8ef5e

This should mast the issue as the bug probably still exists in WMPA path.
Can somebody test that Spitzer fixes it, I cannot get hold on the device?

https://bugs.chromium.org/p/chromium/issues/detail?id=622325 is related bug for Chrome

candrada@: could you, please, assign to the right person to test?

when this is pushed into Android System WebView beta apk-s, please let know. we are currently working on a project that uses "webview via android app", the whole project depends on the ability to play & loop html5 videos (thus this bugreport). plus, we are using both "plain" audio for the video as well as using audio context to manipulate the audio of the video, channel gains/merging/panning a.o. we will happily put it under rigorous field test if possible.

Device -SMP605/KOT49H/53.0.2785.49(Chrome Beta)/52.0.2743.91(Chrome stable)
Test url -http://jsbin.com/fijaxin/4/edit?html,js,output

Scenario 1:
Enable the flag -"Disable the unified media pipeline on Android" -> Video is  playing

Scenario 2:
Disable the flag -"Disable the unified media pipeline on Android" -> Video is not playing 

logs @http://go/chrome-androidlogs1/6/623943

kravula@: please confirm that you see the issue in Chrome (i.e. not only WebView).

I tested in chrome

M52 is default on for Chrome in M52, we're at 45% roll out right now, but will be at 100% in the next week or so. Thus for Chrome this should be resolved, WebView issues I'm not sure about.

I've been surprised by #9, I read it as it works when Spitzer is disabled. And it mentions Chrome 53.0.2785.49 that should be 100% Spitzer? Maybe I miss something.

Okay this is confusing, c#0 is about WebView only, c#9 is reporting things which  c#0 explicitly says are not true, and  issue 622325  tracks a similar problem that is now fixed.

timav@ can you get this device and see what the actual issue is?

I could not get the right device on the first round, but I'll try again.

Got the same device as in comment #9 (SMP605/KOT49H/53.0.2785.49(Chrome Beta)).
The test http://jsbin.com/fijaxin/4/edit?output played for me in Chrome for both enabled and disabled Spitzer.

On this device the jsbin.com site displays check box "Auto-run JS", video played for me only of this box was checked. Then you need to tap the video, and it played.

For non-Spitzer pipeline it seems you need to tap once to have the black screen appear, since there is no preview, and then tap again, and the video played.

It does not seem to me as a problem.

Seems like WontFix to me then.

The original report was on WebView. However, I cannot install a recent system webview on this user KitKat device because it requires API 21 (Lollipop) and I cannot flush this device either.

WontFix.

OP here. I can confirm that video is looping after installing newest WebView beta (SM-P605, 53.0.2785.57) and issues described in the report do not manifest any more. Video loops, audio loops, both in sync, no glitches. Tested with MP4.