Restrict chrome.runtime.setUninstallURL to http(s)

      Disallow URLs other than http(s) in chrome.runtime.setUninstallURL.
And allow empty URLs to be set to clear the uninstallation URL.
Added an optional callback, to know when setting the URL finished (or
failed).

BUG=518827
TEST=browser_tests --gtest_filter=ExtensionApiTest.ChromeRuntimeUninstallURL
R=kalman@chromium.org

Review URL: https://codereview.chromium.org/1282263002

Cr-Commit-Position: refs/heads/master@{#342752}