Only enable Rappor for UMA users

Also, destroy the RapporService during TearDown to clean up URLFetchers properly.

BUG=328168

Review URL: https://codereview.chromium.org/170313007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253024 0039d316-1c4b-4281-b951-d872f2087c98
NaCl documentation: add links for x86-{32,64} and MIPS32.

Also improve explanation for PNaCl.

R= sbc@chromium.org, binji@chromium.org, awatson@chromium.org
TEST= none
BUG= none
NOTRY=true
(documentation only change)

Review URL: https://codereview.chromium.org/176663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253025 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253024
SVN changes up to revision 253025