Adds a webstorePrivate method for silently installing extensions.

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

+<script src="common.js"></script>

[Aaron Boodman, 2011/11/01 17:24:34]

You should test that people can call silentlyInstall() multiple times in quick
succession. I bet that the store team will want to do that to improve UI (so the
download bar doesn't pop up and down repeatedly).

[jstritar, 2011/11/01 19:43:05]

Done. I needed to add a new flag for this, since the way we were running the
other tests only supported 1 CRX file per test. I also added a couple crx files
which are in a second CL (so we can get this through trybots).

+<script>
+
+runTests([
+  function invalidID() {
+    var expectedError = "Invalid id";
+    chrome.webstorePrivate.silentlyInstall(
+        { 'id': appId, 'manifest': getManifest() },
+        callbackFail(expectedError));
+  },
+
+  function successfulInstall() {
+    listenOnce(chrome.management.onInstalled, function(info) {
+      assertEq(info.id, extensionId);
+    });
+
+    chrome.webstorePrivate.silentlyInstall(
+        { 'id': extensionId, 'manifest': getManifest() },
+        callbackPass(function() {
+      checkInstalled(callbackPass(function(result) { assertTrue(result); }));
+    }));
+  }
+]);
+
+</script>