Only generate symbols for binaries that were updated since the last time

To wipe all symbols, you can use the --clear option

BUG=329827
R=dpranke@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/134193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246917 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246917