Preliminary work for unprefixed MSE SourceBuffer.mode manipulation

Adds the SourceBuffer.mode attribute and basic layout tests.
Later changes will plumb this through to correctly process 'segments' and
'sequence' append modes.

R=acolwell@chromium.org
TEST=On Linux, new layout tests pass with no other mediasource layout test regression. SourceBuffer.mode attribute's implementation is correctly hidden: new tests fail in Chromium without --enable-experimental-web-platform-features.
BUG=249422

Review URL: https://codereview.chromium.org/130523002

git-svn-id: svn://svn.chromium.org/blink/trunk@164919 bbb929c8-8fbe-4397-9dbb-9b2b20218538