Initial checkin of accessibility extensions.

ui/accessibility/extensions/alt/manifest.json

+{
+  "name": "Replace images with alt text",
+  "version": "0.1",
+  "description": "Replace images with their alt text",
+  "background": {
+    "scripts": [ "background.js" ]
+  },
+  "commands": {
+    "example_keyboard_command": {
+      "suggested_key": {
+        "default": "Ctrl+Shift+I",
+        "mac": "Ctrl+Shift+I"
+      },
+      "description": "Replace all images with their alt text."
+    }
+  },
+  "browser_action": {
+    "default_title": "Replace all images with their alt text.",
+    "default_icon": {
+      "19": "images/icon-19.png",
+      "38": "images/icon-38.png"
+    }
+  },
+  "options_page": "options.html",
+  "permissions": [
+    "activeTab"
+  ],
+  "manifest_version": 2,
+  "web_accessible_resources": [
+    "images/speech-16.png",
+    "images/speech-missing-alt-16.png",
+    "images/statusbarButtonGlyphs.png"
+  ]
+}