MD Extensions: Fix X button styling

This was missed as part of the v2 paper-icon-button-light migration.

Bug: 822573
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ifeacc6ca78efb16df0de851698bff71d11fac5b5
Reviewed-on: https://chromium-review.googlesource.com/976383Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545538}

chrome/browser/resources/md_extensions/shortcut_input.html

@@ -29,7 +29,7 @@
         };
       }
 
-      #clear {
+      #clearContainer {
         bottom: -4px;
         position: absolute;
         right: -8px;
@@ -45,7 +45,7 @@
           value="[[computeText_(capturing_, shortcut, pendingShortcut_)]]"
           no-label-float>
       </paper-input>
-      <paper-icon-button-light class="icon-clear no-overlap"
+      <paper-icon-button-light class="icon-clear no-overlap" id="clearContainer"
           hidden$="[[computeClearHidden_(capturing_, shortcut)]]">
         <button id="clear" on-click="onClearTap_"></button>
       </paper-icon-button-light>