Enable round-tripping and updating of SourceBuffer timestamp offset

LayoutTests/http/tests/media/media-source/mediasource-sourcebuffer-mode.html

Index: LayoutTests/http/tests/media/media-source/mediasource-sourcebuffer-mode.html
diff --git a/LayoutTests/http/tests/media/media-source/mediasource-sourcebuffer-mode.html b/LayoutTests/http/tests/media/media-source/mediasource-sourcebuffer-mode.html
index c3d41ef8203d5f413a28727ecf967854e7ae00db..33650c3e77de7344912279e1b335d35ecf92a324 100644
--- a/LayoutTests/http/tests/media/media-source/mediasource-sourcebuffer-mode.html
+++ b/LayoutTests/http/tests/media/media-source/mediasource-sourcebuffer-mode.html
@@ -102,7 +102,8 @@
                   assert_equals(sourceBuffer.mode, 'segments');
                   sourceBuffer.mode = 'segments';  // No exception should occur.
                   assert_equals(sourceBuffer.timestampOffset, 0.0);
-                  sourceBuffer.timestampOffset = 10.0;  // No exception should occur.
+                  sourceBuffer.timestampOffset = 10.123456789;  // No exception should occur.
+                  assert_equals(sourceBuffer.timestampOffset, 10.123456789);  // Super-precise offsets should round-trip.
 
                   // Append first part of media segment.
                   test.expectEvent(sourceBuffer, 'updateend', 'Partial media segment append ended.');
@@ -116,7 +117,7 @@
                   assert_throws('InvalidStateError',
                       function() { sourceBuffer.mode = 'segments'; },
                       'Setting valid sourceBuffer.mode while still parsing media segment threw InvalidStateError.');
-                  assert_equals(sourceBuffer.timestampOffset, 10.0);
+                  assert_equals(sourceBuffer.timestampOffset, 10.123456789);
                   assert_throws('InvalidStateError',
                       function() { sourceBuffer.timestampOffset = 20.0; },
                       'Setting valid sourceBuffer.timestampOffset while still parsing media segment threw InvalidStateError.');
@@ -131,7 +132,7 @@
                   assert_false(sourceBuffer.updating, 'updating attribute is false');
                   assert_equals(sourceBuffer.mode, 'segments');
                   sourceBuffer.mode = 'segments';  // No exception should occur.
-                  assert_equals(sourceBuffer.timestampOffset, 10.0);
+                  assert_equals(sourceBuffer.timestampOffset, 10.123456789);
                   sourceBuffer.timestampOffset = 20.0;  // No exception should occur.
                   test.done();
               });