Block modifier-letter-voicing character from domain names

This character (ˬ) is easy to miss between other characters. It's one of the three characters from Spacing-Modifier-Letters block that ICU lists in its recommended set in uspoof.cpp. Two of these characters (modifier-letter-turned-comma and modifier-letter-apostrophe) are already blocked in crbug/678812.

Bug: 896717
Change-Id: I24b2b591de8cc7822cd55aa005b15676be91175e
Reviewed-on: https://chromium-review.googlesource.com/c/1303037
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604128}

components/url_formatter/idn_spoof_checker.cc

@@ -506,6 +506,10 @@ void IDNSpoofChecker::SetAllowedUnicodeSet(UErrorCode* status) {
   // Block {Single,double}-quotation-mark look-alikes.
   allowed_set.remove(0x2bbu);  // Modifier Letter Turned Comma
   allowed_set.remove(0x2bcu);  // Modifier Letter Apostrophe
+
+  // Block modifier letter voicing.
+  allowed_set.remove(0x2ecu);
+
   // No need to block U+144A (Canadian Syllabics West-Cree P) separately
   // because it's blocked from mixing with other scripts including Latin.
 

components/url_formatter/url_formatter_unittest.cc

@@ -1013,7 +1013,11 @@ const IDNTestCase idn_cases[] = {
     // Test that top domains whose skeletons are the same as the domain name are
     // handled properly. In this case, tést.net should match test.net top
     // domain.
-    {"xn--tst-bma.net", L"t\x00e9st.net", false}};
+    {"xn--tst-bma.net", L"t\x00e9st.net", false},
+
+    // Modifier-letter-voicing should be blocked (wwwˬtest.com).
+    {"xn--wwwtest-2be.com", L"www\x02ectest.com", false},
+};
 
 struct AdjustOffsetCase {
   size_t input_offset;