ARM Linux sandbox: make sure r7 is marked as clobbered

Due to a typo, on ARM, the r7 register wasn't marked as clobbered.
(found by rsesek@)

R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/118743003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241931 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241931