Require Renderer::Initialize() to return a status code via callback.

Require Renderer::Initialize() to return a status code via callback.

Using OnError() to deliver status codes during Initialize() complicates
upstream state paths, so require clients to return status via the
initialization callback provided instead of via the error callback.

BUG=450764
TEST=media_unittests, mojo://media_test

Review URL: https://codereview.chromium.org/870693002

Cr-Commit-Position: refs/heads/master@{#312958}