Bug 1535797 [wpt PR 15438] - Update RTCDataChannel bufferedamountlow …

…implementation., a=testonly

Automatic update from web-platform-tests
Update RTCDataChannel bufferedamountlow implementation.

Maintain bufferedamount value in blink layer. Increase bufferedamount on send
and decrease it on OnBufferedAmountChange updates from webrtc layer. Fire
bufferedamountlow when bufferedamount goes below bufferedamountlowthreshold.

This change builds upon the corresponding webrtc change:
https://webrtc-review.googlesource.com/c/src/+/123237

Design doc: http://doc/1lorHBn-GMn5U0T0RQANxrsW0pXhw8XGZM-xZyVUOW90
Standard: https://w3c.github.io/webrtc-pc/#event-datachannel-bufferedamountlow

Bug: 878682
Change-Id: Ifebc79f71f924ecb5cedce1c94e18440663337f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1474764
Commit-Queue: Marina Ciocea <marinacioceachromium.org>
Reviewed-by: Henrik Boström <hboschromium.org>
Cr-Commit-Position: refs/heads/master{#639846}

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wpt-commits: 791052bb65a653296c9fe414e94f5ee1bfb88864
wpt-pr: 15438

UltraBlame original commit: 9b779595e781991c16f9e9314fdbd4fb653e2df2