Fix SourceBuffer.timestampOffset setter behavior for invalid values.

This patch adds logic to SourceBuffer::setTimestampOffset() to reject
invalid values. A LayoutTest to verify proper behavior for a variety of
values was also added.

BUG=353245
TEST=LayoutTests/http/tests/media/media-source/mediasource-timestamp-offset.html

Review URL: https://codereview.chromium.org/202423002

git-svn-id: svn://svn.chromium.org/blink/trunk@169773 bbb929c8-8fbe-4397-9dbb-9b2b20218538