Move preferred stylesheet set out of active sheet update.
There are two reasons for this:
1. In preparation for async active stylesheet update, in which case
setting the preferred stylesheet set would otherwise be happening
too late.
2. Doing it during the active stylesheet update means only the
alternate stylesheets following the link with the title setting the
preferred sheet set would be enabled appropriately. crbug.com/621479
This CL is split out of https://codereview.chromium.org/1913833002
R=timloh@chromium.org
BUG=567021,621479
Review-Url: https://codereview.chromium.org/2079303002
Cr-Commit-Position: refs/heads/master@{#402139}
7 files changed
