Inject automatically on install etc., add mime types, and fix nits.

go-back-with-backspace/background.js

 // Copyright 2016 Google Inc. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Put up an informative message on first install.
+// Inject the content scripts into all open tabs on first install or update.
 chrome.runtime.onInstalled.addListener(function(details) {
-  if (details.reason == "install") {
-    chrome.tabs.create({url: "pages/installed.html"});
-  }
+  if (details.reason == 'install' || details.reason === 'update')
+    injectContentScripts();
 });
+
+// Inject the content scripts into all open tabs when this extension is
+// re-enabled.
+chrome.management.onEnabled.addListener(function(info) {

[Devlin, 2016/09/27 16:08:57]

actually, the more I think about it, does this work?  If the extension was just
enabled, I don't know that we start the background page fast enough to get the
event.

[Pam (message me for reviews), 2016/09/27 23:37:32]

It works when I've tested it. Certainly there might be some race condition I
haven't encountered.

+  if (info.id === chrome.runtime.id)
+    injectContentScripts();
+});
+
+// Listen for messages from the content script, so an old version can detect
+// the loss of connection and disable itself when the extension has been
+// updated, disabled, or uninstalled.
+chrome.runtime.onMessage.addListener(function(message, from, reply) {
+  reply();
+});
+
+// Inject the content scripts into every open tab, on every window.
+function injectContentScripts() {
+  var scripts = chrome.runtime.getManifest().content_scripts[0].js;
+
+  chrome.tabs.query({}, function(tabs) {
+    for (var tab_index = 0; tab_index < tabs.length; ++tab_index) {

[Devlin, 2016/09/27 16:08:57]

suggestion:
for (let tab of tabs)
or
tabs.forEach(function(tab) {

});

[Pam (message me for reviews), 2016/09/27 23:37:32]

Done.

+      var tab = tabs[tab_index];
+      for (var i = 0; i < scripts.length; ++i) {
+        // This will produce an error if extensions are prohibited on the
+        // tab (e.g., chrome:// pages), but we can ignore it.
+        chrome.tabs.executeScript(tab.id,
+                                  {
+                                    file: scripts[i],
+                                    allFrames: true,
+                                    runAt: 'document_start'
+                                  });
+      }
+    }
+  });
+}