Continue over ID-less children in ReactMount.findComponentRoot.

This fixes our perf test by coping with edge cases like the
injection of `<tbody>` between `<table>` and `<tr>` nodes, which occurs
automatically in some browsers when we set `.innerHTML`.

Introducing more search branches would be risky if not for my previous
commit that made `findComponentRoot` breadth-first instead of depth-first.

src/core/ReactInstanceHandles.js

@@ -303,6 +303,13 @@ var ReactInstanceHandles = {
             firstChildren.push(child.firstChild);
             break;
           }
+        } else {
+          // If this child had no ID, then there's a chance that it was
+          // injected automatically by the browser, as when a `<table>`
+          // element sprouts an extra `<tbody>` child as a side effect of
+          // `.innerHTML` parsing. Optimistically continue down this
+          // branch, but not before examining the other siblings.
+          firstChildren.push(child.firstChild);
         }
         child = child.nextSibling;
       }

src/core/__tests__/ReactInstanceHandles-test.js

@@ -133,15 +133,20 @@ describe('ReactInstanceHandles', function() {
       // No ID on `childNodeA`, it was "rendered by the browser".
       ReactID.setID(childNodeB, '.react[0].1:0');
 
+      expect(ReactInstanceHandles.findComponentRoot(
+        parentNode,
+        ReactID.getID(childNodeB)
+      )).toBe(childNodeB);
+
       expect(function() {
         ReactInstanceHandles.findComponentRoot(
           parentNode,
-          ReactID.getID(childNodeB)
+          ReactID.getID(childNodeB) + ":junk"
         );
       }).toThrow(
-        'Invariant Violation: findComponentRoot(..., .react[0].1:0): Unable ' +
-        'to find element. This probably means the DOM was unexpectedly ' +
-        'mutated (e.g. by the browser).'
+        'Invariant Violation: findComponentRoot(..., .react[0].1:0:junk): ' +
+        'Unable to find element. This probably means the DOM was ' +
+        'unexpectedly mutated (e.g. by the browser).'
       );
     });
   });