Add test showing "about:blank" is a unique Origin

This tripped me up for a second so I went looking in unit tests
for documentation.

Bug: None
Change-Id: If2e3c965e98a76d200afb4a1367141ee266ea715
Reviewed-on: https://chromium-review.googlesource.com/1136457Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575235}

url/origin_unittest.cc

@@ -45,6 +45,7 @@ TEST(OriginTest, UniqueOriginComparison) {
 
   const char* const urls[] = {"data:text/html,Hello!",
                               "javascript:alert(1)",
+                              "about:blank",
                               "file://example.com:443/etc/passwd",
                               "yay",
                               "http::///invalid.example.com/"};