Fixing Coverity bugs (DEAD_CODE and PASS_BY_VALUE)

CIDs 16230, 16439, 16610, 16635

BUG=NONE
TEST=NONE

Review URL: http://codereview.chromium.org/7215029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@90134 0039d316-1c4b-4281-b951-d872f2087c98
const ListValue& GpuDataManager::log_messages() const {
  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
  return log_messages_;
}

const ListValue& GpuDataManager::log_messages() const {
  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
  return log_messages_;
}
