Plugin Power Saver: Accept SPACE or ENTER to unthrottle plugins.

Enhances accessibility of throttled plugins.

This enables these steps:
1. Using ChromeVox, focus throttled plugin using Alt+Shift+Arrow Keys.
2. Press ENTER or SPACE.

BUG=458285

Review URL: https://codereview.chromium.org/977653002

Cr-Commit-Position: refs/heads/master@{#318951}

chrome/renderer/resources/plugin_poster.html

@@ -8,6 +8,13 @@ function notifyDidFinishLoading() {
   if (plugin.didFinishLoading)
     plugin.didFinishLoading();
 }
+
+document.onkeydown = function(e) {
+  if (e.keyIdentifier == 'Enter' || e.keyIdentifier == 'U+0020') {
+    plugin.load();
+    e.preventDefault();
+  }
+};
 </script>
 <link rel="stylesheet" href="plugin_placeholders.css"></link>
 <style>