MD Extensions polish: Change background color of installation warning.

Bug: 794003
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I4dc96f9a899ccc9b0b208f2f326772208b47c187
Reviewed-on: https://chromium-review.googlesource.com/822974Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523698}

chrome/browser/resources/md_extensions/install_warnings_dialog.html

@@ -14,7 +14,7 @@
     <style include="cr-shared-style paper-button-style">
       div[slot='body'] ul {
         -webkit-padding-end: 10px;
-        background-color: var(--paper-pink-50);
+        background-color: var(--paper-red-50);
         margin: 0;
         padding-bottom: 10px;
         padding-top: 10px;