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In order to support impl-side painting plus an embedder without a
message loop, cc needs a setting to disable raster.  Rastering with
impl-side painting requires a task runner (and DCHECKs this).  This
setting notifies cc that there's no need for a tile manager.

Thankfully, tile manager is already considered optional in almost all
contexts, and so just a few more callsites needed to be cleaned up to
fix a few crashes.

With this patch, all cc unittests pass with impl-side painting on by
default.

Review URL: https://codereview.chromium.org/984113004

Cr-Commit-Position: refs/heads/master@{#319840}