Initialize mountUrl_ in Add Share Dialog so it is not undefined

- After switching to cr-searchable-drop-down, mountUrl_ was undefined, this
switches it to being explicitly initialized to an empty string so the Add
button is correctly disabled.

Bug: chromium:882590
Change-Id: I38dcc3a17b9406f2d633b90154114ad20876fa91
Reviewed-on: https://chromium-review.googlesource.com/1217514
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590128}

chrome/browser/resources/settings/downloads_page/add_smb_share_dialog.js

@@ -13,17 +13,28 @@ Polymer({
 
   properties: {
     /** @private {string} */
-    mountUrl_: String,
+    mountUrl_: {
+      type: String,
+      value: '',
+    },
 
     /** @private {string} */
-    mountName_: String,
+    mountName_: {
+      type: String,
+      value: '',
+    },
 
     /** @private {string} */
-    username_: String,
+    username_: {
+      type: String,
+      value: '',
+    },
 
     /** @private {string} */
-    password_: String,
-
+    password_: {
+      type: String,
+      value: '',
+    },
     /** @private {!Array<string>}*/
     discoveredShares_: {
       type: Array,