Replace innerHTML assignments with safer versions.

In these cases, it doesn't really matter; assigning constant strings is
generally safe, but it's nice to avoid innerHTML assignments anyway, I guess.

Submitted on behalf of a Googler.

Author: juangj

javascript/atoms/test/focus_test.html

@@ -247,7 +247,7 @@
         });
       goog.events.listen(goog.dom.$('clearTestLog'), eventType.CLICK,
         function(e) {
-          goog.dom.$('testLog').innerHTML = '';
+          goog.dom.removeChildren(goog.dom.$('testLog'));
         });
     </script>
   </div>

javascript/atoms/test/shown_test.html

@@ -341,9 +341,9 @@
     function testOverflowHiddenWithFixedPositionChild() {
       var target = findElement({id: 'overflowHiddenWithFixedPositionChild'});
       var child = findElement({id: 'fixedPositionChild'});
-      child.innerHTML = '';
+      goog.dom.removeChildren(child);
       assertFalse(isShown(target));
-      child.innerHTML = 'some content';
+      goog.dom.setTextContent(child, 'some content');
       assertTrue(isShown(target));
     }