Fixes incorrect URL updates for registered schemes (e.g. "steam:")

Updating url.pathname of non-special scheme URLs whose schemes are
internally registered as opaque non-special schemes (e.g "steam:") adds
a leading slash wrongly.

This issue was found by a gurl_fuzzer.

The CL fixes their behaviors as follows:

Before:

> const url = new URL("steam:a")
> url.pathname = "b"
> url.href
"steam:/b"

After:

> const url = new URL("steam:a")
> url.pathname = "b"
> url.href
"steam:b"

Every other normal non-special scheme URL, like "git:a", doesn't have
this issue.

Bug: 40063064,375660989
Change-Id: I3a8104f48acf956747e0efa0cd7d612fe26a26e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5975076
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1376010}

url/gurl_unittest.cc

@@ -829,6 +829,18 @@ TEST(GURLTest, Replacements) {
              return url.ReplaceComponents(replacements);
            },
        .expected = "filesystem:http://www.google.com/foo/bar.html?foo#bar"},
+      // Regression test for https://crbug.com/375660989.
+      //
+      // "steam:" is explicitly registered as an opaque non-special scheme for
+      // compatibility reasons. See SchemeRegistry::opaque_non_special_schemes.
+      {.base = "steam:a",
+       .apply_replacements =
+           +[](const GURL& url) {
+             GURL::Replacements replacements;
+             replacements.SetPathStr("b");
+             return url.ReplaceComponents(replacements);
+           },
+       .expected = "steam:b"},
   };
 
   for (const ReplaceCase& c : replace_cases) {

url/url_util.cc

@@ -532,7 +532,8 @@ bool DoReplaceComponents(const char* spec,
     return ReplaceMailtoURL(spec, parsed, replacements, output, out_parsed);
   }
 
-  if (IsUsingStandardCompliantNonSpecialSchemeURLParsing()) {
+  if (IsUsingStandardCompliantNonSpecialSchemeURLParsing() &&
+      !DoIsOpaqueNonSpecial(spec, parsed.scheme)) {
     return ReplaceNonSpecialURL(spec, parsed, replacements, charset_converter,
                                 *output, *out_parsed);
   }