Use IDL enum to ignore invalid SourceBuffer mode attribute setting

Adheres to current MediaSource's SourceBuffer mode attribute spec that
uses a WebIDL enum for AppendMode. In this case, scripts' attempts to
set invalid mode attribute values outside the range of the enum now
result in the attempts being ignored, as WebIDL spec requires.
Updates mediasource-sourcebuffer-mode layout test accordingly.

R=acolwell@chromium.org
BUG=249422
TEST=Updated layout test passes on local linux.

Review URL: https://codereview.chromium.org/156853005

git-svn-id: svn://svn.chromium.org/blink/trunk@166735 bbb929c8-8fbe-4397-9dbb-9b2b20218538