Make SMB add share dialog fields full width

- Makes add dialog share fields stretch the full width of the dialog
- Adds width styling mix-in to cr-searchable-drop-down
- Updates add printer styling to be consistent after removing fixed
width styling from cr-searchable-drop-down

Bug: chromium:887111
Change-Id: Ibcb9c67b90cff9d0c5bf0c8cc058b6d218cb1f1a
Reviewed-on: https://chromium-review.googlesource.com/1244614
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594516}

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

@@ -19,11 +19,11 @@
 
       cr-input {
         --cr-input-error-display: none;
-        width: var(--settings-input-max-width);
       }
 
       cr-searchable-drop-down {
         display: block;
+        --cr-searchable-drop-down-width: 472px;
       }
 
       cr-input:not(:last-child),

ui/webui/resources/cr_elements/cr_searchable_drop_down/cr_searchable_drop_down.html

@@ -13,7 +13,7 @@
 
       iron-dropdown,
       cr-input {
-        width: 270px;
+        width: var(--cr-searchable-drop-down-width, 270px);
       }
 
       iron-dropdown {