WebUI cr-dialog: Prevent text from being selected when clicking backdrop.

Bug: 742727
Change-Id: Ia6bacffad903f7fd9d5a9e7d460dde293214b671
Reviewed-on: https://chromium-review.googlesource.com/572205Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487201}

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js

@@ -222,5 +222,9 @@ Polymer({
           easing: 'ease-in-out',
           iterations: 1,
         }));
+
+    // Prevent any text from being selected within the dialog when clicking in
+    // the backdrop area.
+    e.preventDefault();
   },
 });