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 <marinaciocea@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639846}

Author: cdmarina

webrtc/RTCDataChannel-bufferedAmount.html

@@ -166,4 +166,59 @@
       assert_unreached(`Unexpected promise rejection: ${err}`)));
   }, 'bufferedAmount should increase by byte length for each message sent');
 
+  promise_test(async t => {
+    const [channel1, channel2] = await createDataChannelPair();
+    channel1.addEventListener('bufferedamountlow', t.step_func_done(() => {
+      assert_true(channel1.bufferedAmount <= channel1.bufferedAmountLowThreshold);
+    }));
+    const eventWatcher = new EventWatcher(t, channel1, ['bufferedamountlow']);
+    channel1.send(helloString);
+    await eventWatcher.wait_for(['bufferedamountlow']);
+  }, 'Data channel bufferedamountlow event fires after send() is complete');
+
+  promise_test(async t => {
+    const [channel1, channel2] = await createDataChannelPair();
+    channel1.send(helloString);
+    assert_equals(channel1.bufferedAmount, helloString.length);
+    await awaitMessage(channel2);
+    assert_equals(channel1.bufferedAmount, 0);
+  }, 'Data channel bufferedamount is data.length on send(data)');
+
+  promise_test(async t => {
+    const [channel1, channel2] = await createDataChannelPair();
+    channel1.send(helloString);
+    assert_equals(channel1.bufferedAmount, helloString.length);
+    assert_equals(channel1.bufferedAmount, helloString.length);
+  }, 'Data channel bufferedamount returns the same amount if no more data is' +
+     ' sent on the channel');
+
+  promise_test(async t => {
+    const [channel1, channel2] = await createDataChannelPair();
+    let eventFireCount = 0;
+    channel1.addEventListener('bufferedamountlow', t.step_func(() => {
+      assert_true(channel1.bufferedAmount <= channel1.bufferedAmountLowThreshold);
+      assert_equals(++eventFireCount, 1);
+    }));
+    const eventWatcher = new EventWatcher(t, channel1, ['bufferedamountlow']);
+    channel1.send(helloString);
+    assert_equals(channel1.bufferedAmount, helloString.length);
+    channel1.send(helloString);
+    assert_equals(channel1.bufferedAmount, 2 * helloString.length);
+    await eventWatcher.wait_for(['bufferedamountlow']);
+  }, 'Data channel bufferedamountlow event fires only once after multiple' +
+    ' consecutive send() calls');
+
+  promise_test(async t => {
+    const [channel1, channel2] = await createDataChannelPair();
+    const eventWatcher = new EventWatcher(t, channel1, ['bufferedamountlow']);
+    channel1.send(helloString);
+    assert_equals(channel1.bufferedAmount, helloString.length);
+    await eventWatcher.wait_for(['bufferedamountlow']);
+    assert_equals(await awaitMessage(channel2), helloString);
+    channel1.send(helloString);
+    assert_equals(channel1.bufferedAmount, helloString.length);
+    await eventWatcher.wait_for(['bufferedamountlow']);
+    assert_equals(await awaitMessage(channel2), helloString);
+  }, 'Data channel bufferedamountlow event fires after each sent message');
+
 </script>