Add layout states to DocumentLifecycle state machine

This CL adds a number of layout-related states to the DocumentLifecycle state
machine. Currently, we're using these states to ASSERT that we're making valid
transitions. Eventually, we'll switch over to these states as the source of
truth.

R=ojan@chromium.org
BUG=341627

Review URL: https://codereview.chromium.org/146023008

git-svn-id: svn://svn.chromium.org/blink/trunk@166765 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unused parts of MediaControllerInterface

Remove the parts of MediaControllerInterface that aren't actually used
by the MediaControls code, which is the only place where this abstract
interface is used as opposed to HTMLMediaElement or MediaController.

hasCurrentSrc() wasn't used anywhere, so remove it.

BUG=341852

Review URL: https://codereview.chromium.org/156173003

git-svn-id: svn://svn.chromium.org/blink/trunk@166766 bbb929c8-8fbe-4397-9dbb-9b2b20218538