Introduce WebMediaPlayer::seekable() to replace maxTimeSeekable()

TEST=LayoutTests/media/video-seekable.html

Update the test to check that seekable end time is between 5 and 7
seconds, as content/test.* are around 6 seconds long. This should
catch grossly incorrect implementations of seekable.

Also wait only for loadedmetadata, as that should be enough to
determine the seekable ranges, and an implementation waiting until
canplaythrough should fail the test.

BUG=417669

Review URL: https://codereview.chromium.org/607493002

git-svn-id: svn://svn.chromium.org/blink/trunk@182821 bbb929c8-8fbe-4397-9dbb-9b2b20218538