Have Player take the DocumentTimeline object by reference

Have Player take the DocumentTimeline object by reference instead of using a pointer
with an assertion to make sure it is non-null. This also makes the code using the
DocumentTimeline look safer.

R=adamk

Review URL: https://codereview.chromium.org/48523004

git-svn-id: svn://svn.chromium.org/blink/trunk@160737 bbb929c8-8fbe-4397-9dbb-9b2b20218538