fix(login-fire-button): ensure child has same width

Before this commit the `paper-social-button` width could not be the same
as its parent. So when user was setting the width of `login-fire-button`
the button was not applying it. Now, it does so you can change the width
of the button directly on the `login-fire-button` and not only on the
encapsulated `paper-social-button`.

Author: masonlouchart

login-fire-button.html

@@ -42,6 +42,14 @@
 -->
 <dom-module id="login-fire-button">
   <template>
+    <style>
+    :host {
+      display: inline-block;
+      --login-fire-button: {
+        width: 100%;
+      };
+    }
+    </style>
     <paper-social-button
       provider="[[config.provider]]"
       no-sign-in="[[noSignIn]]"