Use simple HAS-A relationship instead of obscure multiple inheritance.
In theory, UBSan should be totally happy with this, but it still isn't.
This seems to be a bug in UBSan, see e.g.
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-December/040225.html

BUG=chromium:448102
LOG=y

Review URL: https://codereview.chromium.org/859773002

Cr-Commit-Position: refs/heads/master@{#26139}