Address ExtensionApiTest.Tabs flakiness

This addresses two problems. (1) relativeUrlTabsUpdate could fail because it was depending on testTabId getting set from relativeUrlTabsCreate, which may have happened out of order. I have refactored the callback blocking so additional blocks can be created with chrome.test.callbackAdded().

(2) tabs.update({url:}) is still failing intermitantly. This changes addeds valid html files in the extension which can be navitation targets (previously, non-existent urls had been used), in the hopes that the random failures had to do with load errors.

BUG=20828

Review URL: http://codereview.chromium.org/195090

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26258 0039d316-1c4b-4281-b951-d872f2087c98