Media controls should fade out after a while regardless of mouse position

Media controls should fade out after 15sec regardless of the mouse position. Incase of fullscreen controls the above mentioned behavior is already implemented. 
The below patch implements same behavior for inline media controls.

BUG=151691

Review URL: https://codereview.chromium.org/212173002

git-svn-id: svn://svn.chromium.org/blink/trunk@170953 bbb929c8-8fbe-4397-9dbb-9b2b20218538