Exclude unused performance_monitor strings from Android builds.

Performance monitor page is not used on Android and so all the
performance monitor strings are unused.

BUG=326023
R=oysteine@chromium.org, yoz@chromium.org

Review URL: https://codereview.chromium.org/147783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247290 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247290