[MD settings] prevent paper-buttons from shrinking

This CL prevents flex pressure from squeezing paper-buttons. E.g. prevent
a long account name from making the "sign in to chrome" button into a
column of text.

BUG=718318
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2864483006
Cr-Commit-Position: refs/heads/master@{#469502}

chrome/browser/resources/settings/settings_shared_css.html

@@ -36,6 +36,7 @@
       }
 
       paper-button {
+        flex-shrink: 0;
         height: 36px;
         margin: 0;
       }