MSE: Fix mediasource-changetype-play to work with MseBufferByPts

This web-platform-test exercises changeType as it splice-overlaps
pairs of audio or video media streams at varying offsets in the
presentation timeline. Splice-overlapping an out-of-order decode stream
(such as the test AVC MP4 media) at arbitrary times can, per spec, drop
significant decode dependencies from a partially-overlapped GOP such
that a buffered range gap could result.

This change is more careful about where it performs splice-overlaps when
the overlapped media is out-of-order-decode, adjusting the splice point
to be at or very near to the next overlapped keyframe. This prevents
removing out-of-order non-keyframes and their dependents from the
overlapped media such that no buffered range gap nor playback stall
should result.

Note that Chromium is sensitive to such out-of-order buffering overlaps
with the new, compliant, MseBufferByPts behavior. Fixing
w3c/media-source#160 could greatly simplify
this problem by allowing apps to explicitly control how the
user agent behaves at these small gaps.

BUG=807793

Change-Id: I020e244c230756eaa1804f81b58a577124a6a28b

media-source/mediasource-changetype-play.html

@@ -12,27 +12,26 @@
         <div id="log"></div>
         <script>
 
-function generateChangeTypeTests(audio_types, video_types)
-{
-    async_test(function(test) {
-        assert_true(audio_types.length > 1, "Browser doesn't support at least 2 types of audio test media" + audio_types);
-        assert_true(video_types.length > 1, "Browser doesn't support at least 2 types of video test media");
-        test.done();
-    }, "Check if browser supports enough test media types");
+function generateChangeTypeTests(audio_types, video_types) {
+  async_test((test) => {
+    assert_true(audio_types.length > 1, "Browser doesn't support at least 2 types of audio test media");
+    assert_true(video_types.length > 1, "Browser doesn't support at least 2 types of video test media");
+    test.done();
+  }, "Check if browser supports enough test media types");
 
-    // Generate audio-only changeType tests
-    for (let audio1 of audio_types) {
-        for (let audio2 of audio_types) {
-            mediaSourceChangeTypeTest(audio1, audio2, "Test audio-only changeType for " + audio1.type + " <-> " + audio2.type);
-        }
+  // Generate audio-only changeType tests
+  for (let audio1 of audio_types) {
+    for (let audio2 of audio_types) {
+      mediaSourceChangeTypeTest(audio1, audio2, "Test audio-only changeType for " + audio1.type + " <-> " + audio2.type);
     }
+  }
 
-    // Generate video-only changeType tests
-    for (let video1 of video_types) {
-        for (let video2 of video_types) {
-            mediaSourceChangeTypeTest(video1, video2, "Test video-only changeType for " + video1.type + " <-> " + video2.type);
-        }
+  // Generate video-only changeType tests
+  for (let video1 of video_types) {
+    for (let video2 of video_types) {
+      mediaSourceChangeTypeTest(video1, video2, "Test video-only changeType for " + video1.type + " <-> " + video2.type);
     }
+  }
 }
 
 findSupportedChangeTypeTestTypes(generateChangeTypeTests);