Adds a webstorePrivate method for silently installing extensions.

chrome/test/data/extensions/api_test/webstore_private/silently_install.html

+<script src="common.js"></script>
+<script>
+
+var extension1 = {
+  'id': 'bmfoocgfinpmkmlbjhcbofejhkhlbchk',
+  'manifest':
+    '{\
+      "name": "Extension Bundle 1",\
+      "version": "1",\
+      "permissions": [ "tabs" ]\
+     }'
+};
+
+var extension2 = {
+  'id': 'mpneghmdnmaolkljkipbhaienajcflfe',
+  'manifest':
+    '{\
+      "name": "Extension Bundle 2",\
+      "version": "1",\
+      "permissions": ["management", "http://google.com" ],\
+      "content_script": [{\
+        "matches": [ "http://www.example.com/*" ],\
+        "js": [ "content_script.js" ],\
+        "run_at": "document_start"\
+       }]\
+    }'
+};
+
+var extension3 = {
+  'id': 'begfmnajjkbjdgmffnjaojchoncnmngg',
+  'manifest':
+    '{\
+      "name": "Bundle App 2",\
+      "version": "1",\
+      "app": {\
+        "urls": [ "http://www.testapp2.com" ],\
+        "launch": { "web_url": "http://www.testapp2.com" }\
+      }\
+    }'
+};
+
+runTests([
+  function invalidID() {
+    var expectedError = "Invalid id";
+    chrome.webstorePrivate.silentlyInstall(
+        { 'id': 'dladmdjkfniedhfhcfoefgojhgaiaccc', 'manifest': getManifest() },
+        callbackFail(expectedError));
+  },
+
+  function successfulInstall() {
+    chrome.webstorePrivate.silentlyInstall(extension1, callbackPass(function() {
+      checkItemInstalled(
+          extension1.id,
+          callbackPass(function(result) { assertTrue(result); }));
+    }));
+
+    chrome.webstorePrivate.silentlyInstall(extension2, callbackPass(function() {
+      checkItemInstalled(
+           extension2.id,
+           callbackPass(function(result) { assertTrue(result); }));
+      chrome.webstorePrivate.silentlyInstall(
+          extension3, callbackPass(function() {
+        checkItemInstalled(
+             extension3.id,
+             callbackPass(function(result) { assertTrue(result); }));
+      }));
+    }));
+  }
+]);
+
+</script>