[Extension Doc] Example Command Override

Sample extension that uses command API to give user alternate
choice of keyboard shortcuts to switch between tabs.
Also moves existing sample command extension into a new
directory.

Bug: None
Change-Id: Ia5c4ae7134c51a1730260a5f23f23c091231ff38
Reviewed-on: https://chromium-review.googlesource.com/749115
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521881}

chrome/common/extensions/docs/examples/api/default_command_override/background.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+chrome.commands.onCommand.addListener(function(command) {
+  chrome.tabs.query({currentWindow: true}, function(tabs) {
+    // Sort tabs according to their index in the window.
+    tabs.sort((a, b) => { return a.index < b.index; });
+    let activeIndex = tabs.findIndex((tab) => { return tab.active; });
+    let lastTab = tabs.length - 1;
+    let newIndex = -1;
+    if (command === 'flip-tabs-forward')
+      newIndex = activeIndex === 0 ? lastTab : activeIndex - 1;
+    else  // 'flip-tabs-backwards'
+      newIndex = activeIndex === lastTab ? 0 : activeIndex + 1;
+    chrome.tabs.update(tabs[newIndex].id, {active: true, highlighted: true});
+  });
+});

chrome/common/extensions/docs/examples/api/default_command_override/manifest.json

+{
+  "name": "Tab Flipper",
+  "description": "Press Ctrl+Shift+Right or Ctrl+Shift+Left (Command+Shift+Right or Command+Shift+Left on a Mac) to flip through window tabs",
+  "version": "1.0",
+  "manifest_version": 2,
+  "background": {
+    "scripts": ["background.js"],
+    "persistent": false
+  },
+  "browser_action": {
+    "default_icon": "images/tabFlipper16.png",
+    "default_title": "Press Ctrl(Win)/Command(Mac)+Shift+ Left or Right to Flip Tabs"
+  },
+  "commands": {
+    "flip-tabs-forward": {
+      "suggested_key": {
+        "default": "Ctrl+Shift+Right",
+        "mac": "Command+Shift+Right"
+      },
+      "description": "Flip tabs forward"
+    },
+    "flip-tabs-backwards": {
+      "suggested_key": {
+        "default": "Ctrl+Shift+Left",
+        "mac": "Command+Shift+Left"
+      },
+      "description": "Flip tabs backwards"
+    }
+  },
+  "icons": {
+    "16": "images/tabFlipper16.png",
+    "32": "images/tabFlipper32.png",
+    "48": "images/tabFlipper48.png",
+    "128": "images/tabFlipper128.png"
+  }
+}