Don't enable bottom toolbar button if it is empty

Bug: 877437
Change-Id: Id444baf870a3b9b4942f57bedb0d3b275e29ba81
Reviewed-on: https://chromium-review.googlesource.com/1191133Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586384}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/ToolbarButtonSlotData.java

@@ -109,7 +109,7 @@ class ToolbarButtonSlotData {
 
                 @Override
                 public void onAnimationEnd(Animator animator) {
-                    imageButton.setEnabled(true);
+                    if (mDrawable != null) imageButton.setEnabled(true);
                 }
             });