Better tests for currentSrc

This CL adds a test to make sure redirected URLs are not exposed in currentSrc.
It also slightly modifies the test, to make it more robust.

BUG=378188

Review URL: https://codereview.chromium.org/312683003

git-svn-id: svn://svn.chromium.org/blink/trunk@175373 bbb929c8-8fbe-4397-9dbb-9b2b20218538