MD Downloads: fix up some Polymer 1.2.3 issues and re-vulcanize

chrome/browser/resources/md_downloads/manager.css

Index: chrome/browser/resources/md_downloads/manager.css
diff --git a/chrome/browser/resources/md_downloads/manager.css b/chrome/browser/resources/md_downloads/manager.css
index c7f50d1861f556f3e6b1e8d9a45d724208e0c8b3..5d0891012991d810ae2fe8a4745c885a236d0b17 100644
--- a/chrome/browser/resources/md_downloads/manager.css
+++ b/chrome/browser/resources/md_downloads/manager.css
@@ -7,6 +7,7 @@
   flex: 1 0;
   flex-direction: column;
   height: 100%;
+  overflow-y: overlay;
 }
 
 @media screen and (max-width: 1024px) {
@@ -15,23 +16,13 @@
   }
 }
 
-#panel {
-  --paper-header-panel-shadow: {
-    display: none;
-  };
-  --paper-header-panel-standard-container: {
-    display: flex;
-    overflow-y: overlay;
-  };
-}
-
 #no-downloads,
 #downloads-list {
   flex: 1;
 }
 
-.loading #no-downloads,
-.loading #downloads-list {
+:host([loading]) #no-downloads,
+:host([loading]) #downloads-list {
   display: none;
 }