Only do PNaCl CU OnDemand resource throttles when needed.

Add a pnacl-specific callback to check if it's needed or not.

BUG=315115
R=cpu@chromium.org, darin@chromium.org

Review URL: https://codereview.chromium.org/92393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239943 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239943