Add content ID scheme to //url/url_constants.*

This CL adds the content ID scheme to //url/url_constants.*. This scheme is
specified in https://tools.ietf.org/html/rfc2392. The concrete motivation for
this CL is that an upcoming CL needs to reference the content ID scheme in
order to correctly handle navigations within MHTML documents when using
browser-side navigation.

BUG=623553

Review-Url: https://codereview.chromium.org/2098723003
Cr-Commit-Position: refs/heads/master@{#402185}
diff --git a/url/gurl_unittest.cc b/url/gurl_unittest.cc
index 4378db9..1a0987e 100644
--- a/url/gurl_unittest.cc
+++ b/url/gurl_unittest.cc
@@ -660,6 +660,9 @@
 
   GURL c("foo://bar/baz");
   EXPECT_FALSE(c.IsStandard());
+
+  GURL d("cid:bar@baz");
+  EXPECT_FALSE(d.IsStandard());
 }
 
 TEST(GURLTest, SchemeIsHTTPOrHTTPS) {