Calendar Picker: Make month popup cells non-focusable.

Source/web/resources/calendarPicker.css

Index: Source/web/resources/calendarPicker.css
diff --git a/Source/web/resources/calendarPicker.css b/Source/web/resources/calendarPicker.css
index a67b3189bc5af5513f3c61ee4db86833f54a0e9c..e5cbb72003bb88cd4c66b541c6657eecd7c364de 100644
--- a/Source/web/resources/calendarPicker.css
+++ b/Source/web/resources/calendarPicker.css
@@ -244,18 +244,24 @@ body {
 .month-button {
     flex: 1;
     height: 32px;
+    line-height: 32px;
     padding: 0 !important;
     margin: 0 !important;
     background-image: none !important;
     background-color: #ffffff;
     border-width: 0 !important;
     box-shadow: none !important;
+    text-align: center;
 }
 
 .month-button.highlighted {
     background-color: #e5ecf8;
 }
 
+.month-button[aria-disabled="true"] {
+    color: GrayText;
+}
+
 .scrubby-scroll-bar {
     width: 14px;
     height: 60px;