Pepper: Move more UMA stuff out of trusted plugin.

This is yet another pretty simple change just lifting more lines out of the
trusted plugin. I added a method to PPB_NaCl_Private that's safe to call from
a background thread (there are other methods that are safe as well) to keep the
call sites simple.

BUG=239656
R=dmichael@chromium.org

Review URL: https://codereview.chromium.org/331973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@277424 0039d316-1c4b-4281-b951-d872f2087c98
7 files changed