Drop all timeline seeks outside of the seekable range.

Prevents annoying accidental seeks to the beginning of content for
streaming content; non-streaming content always exposes (0, duration)
as the seekable range so it's unaffected by this change.

BUG=412562
TEST=new layout test.

Review URL: https://codereview.chromium.org/614263002

git-svn-id: svn://svn.chromium.org/blink/trunk@183841 bbb929c8-8fbe-4397-9dbb-9b2b20218538