Update Media Router button style.

This change removes the margin on either side of buttons in the Media Router WebUI.

Review URL: https://codereview.chromium.org/1133283005

Cr-Commit-Position: refs/heads/master@{#330268}

chrome/browser/resources/media_router/media_router_common.css

@@ -9,6 +9,7 @@
   cursor: pointer;
   font-size: 1em;
   font-weight: bold;
+  margin: 0;
   overflow: hidden;
   text-align: center;
   width: 100%;