[Mac] Chrome print dialog disregards system cancel key-command.

Add support for Cmd- (the universal "cancel" shortcut on Mac) to close print dialog.

BUG=249786
R=dpapad@chromium.org

TEST=
1. Open a web page
2. Hit Cmd-P to bring up print dialog.
3. Hit Cmd- and observe.
4. The print dialog should get closed.

Review URL: https://codereview.chromium.org/555733003

Cr-Commit-Position: refs/heads/master@{#294142}

chrome/browser/resources/print_preview/print_preview.js

@@ -855,6 +855,13 @@ cr.define('print_preview', function() {
         return;
       }
 
+      // On Mac, Cmd- should close the print dialog.
+      if (cr.isMac && e.keyCode == 189 && e.metaKey) {
+        this.close_();
+        e.preventDefault();
+        return;
+      }
+
       // Ctrl + Shift + p / Mac equivalent.
       if (e.keyCode == 80) {
         if ((cr.isMac && e.metaKey && e.altKey && !e.shiftKey && !e.ctrlKey) ||