Move PluginList to content/browser/.

It does not need to live in content/common/. Run git cl format along the
way.

Change-Id: I04e063334b7df182f0c5fa4eb235e61f49e77beb
Reviewed-on: https://chromium-review.googlesource.com/c/1456947
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629754}
7 files changed