Files.app: Prevent a scroolbar from overshooting the border of the view.

BUG=245053
TEST=manual
R=mtomasz@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15761006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203956 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/file_manager/js/scrollbar.js

@@ -152,6 +152,9 @@ ScrollBar.prototype.onMouseMove_ = function(event) {
 
   var buttonPosition = this.buttonPressedPosition_ +
       (event.screenY - this.buttonPressedEvent_.screenY);
+  // Ensures the scrollbar is in the view.
+  buttonPosition =
+      Math.max(0, Math.min(buttonPosition, clientSize - buttonSize));
   var scrollPosition = totalSize * (buttonPosition / clientSize);
 
   this.scrollTop_ = scrollPosition;