Fix navigation for the discards tab in chrome://discards.

Special-casing the 'Discards' tab to the empty tab makes the initial
page load navigation entry consistent with entry on back navigation.

Bug: 1070554
Change-Id: Ib1d6f271e12d97db31cd03708f89b41a26009e64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148539
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758979}

chrome/browser/resources/discards/discards_main.js

@@ -45,7 +45,10 @@ Polymer({
    */
   selectedChanged_(newValue, oldValue) {
     if (oldValue != undefined) {
-      this.path = '/' + this.tabs[newValue].toLowerCase();
+      // The first tab is special-cased to the empty path.
+      const defaultTab = this.tabs[0].toLowerCase();
+      const tabName = this.tabs[newValue].toLowerCase();
+      this.path = '/' + (tabName === defaultTab ? '' : tabName);
     }
   },