Bug 1565071 [wpt PR 17776] - [Media Session] Implement "seekto" in Blink, a=testonly

Automatic update from web-platform-tests
[Media Session] Implement "seekto" in Blink

Implement "seekto" action for media session in Blink.

Also implements MediaSessionActionDetails which were
added as part of the same spec change.

BUG=977375

Change-Id: I8a95ebfd8390922e152117d26c2cbb20889c0c9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696204
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679198}

--

wpt-commits: 9501ee7f29d8ad85fe2ea4fd156b5631c0e0eac8
wpt-pr: 17776

testing/web-platform/tests/mediasession/setactionhandler.html

@@ -12,6 +12,7 @@ test(function(t) {
   window.navigator.mediaSession.setActionHandler("seekbackward", null);
   window.navigator.mediaSession.setActionHandler("seekforward", null);
   window.navigator.mediaSession.setActionHandler("stop", null);
+  window.navigator.mediaSession.setActionHandler("seekto", null);
 }, "Test that setActionHandler() can be executed for supported actions");
 
 test(function(t) {