Add a preference to force enable the NaCl plugin as we transition to

Add a preference to force enable the NaCl plugin as we transition to
on-by-default.

This is a mirror change of http://codereview.chromium.org/7720021/ for trunk (some files have changed so it was necessary to check into the branch and trunk separately instead of merging)

BUG=93984
TEST=manual
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=98371

[elijahtaylor (use chromium), 2011-08-24 23:35:15]

Largely the same content as the other change to the M14 branch, but the plugin
prefs changes are in a different file and use slightly different code to deal
with the prefs.

[jam, 2011-08-24 23:39:35]

lgtm

[commit-bot: I haz the power, 2011-08-25 02:28:11]

The commit queue went berserk retrying too often for a
seemingly flaky test. Builder is linux, revision is 98143, job name
was 7693030-1 (retry) (retry) (retry) (retry).