Do not expand languages that starts with the primary subtag 'x' or 'i'

Any language that starts with the  primary language subtag 'x' is
defined by RFC 5646 section 2.2.1 as being defined by private
agreement. Therefore, 'x' isn't a language. Similarly, language tags
starting with the primarty subtag 'i' such as "i-klingon" are not of
the form "language-country" either.

Bug: chromium:1179073
Test: unit tests
Change-Id: I67d0b090df6941339fbbd0d4337d8db0288d74f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2699487
Auto-Submit: Yves Arrouye <drcrash@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#854754}

net/http/http_util.cc

@@ -787,11 +787,17 @@ std::string HttpUtil::ExpandLanguageList(const std::string& language_prefs) {
     const std::string& language = languages[i];
     builder.AddLanguageCode(language);
 
-    // Extract the base language
+    // Extract the primary language subtag.
     const std::string& base_language = GetBaseLanguageCode(language);
 
-    // Look ahead and add the base language if the next language is not part
-    // of the same family.
+    // Skip 'x' and 'i' as a primary language subtag per RFC 5646 section 2.1.1.
+    if (base_language == "x" || base_language == "i")
+      continue;
+
+    // Look ahead and add the primary language subtag as a language if the next
+    // language is not part of the same family. This may not be perfect because
+    // an input of "en-US,fr,en" will yield "en-US,en,fr,en" and later make "en"
+    // a higher priority than "fr" despite the original preference.
     const size_t j = i + 1;
     if (j >= size || GetBaseLanguageCode(languages[j]) != base_language) {
       builder.AddLanguageCode(base_language);

net/http/http_util_unittest.cc

@@ -1633,6 +1633,12 @@ TEST(HttpUtilTest, ExpandLanguageList) {
             HttpUtil::ExpandLanguageList("en-US,fr-CA,it,fr,es-AR,it-IT"));
   // Trims a whitespace.
   EXPECT_EQ("en-US,en,fr", HttpUtil::ExpandLanguageList("en-US, fr"));
+
+  // Do not expand the single character subtag 'x' as a language.
+  EXPECT_EQ("x-private-agreement-subtags",
+            HttpUtil::ExpandLanguageList("x-private-agreement-subtags"));
+  // Do not expand the single character subtag 'i' as a language.
+  EXPECT_EQ("i-klingon", HttpUtil::ExpandLanguageList("i-klingon"));
 }
 
 }  // namespace net