[Intl] add 'b' and 'B' to the skip list.

Mentioned by ICU guru during design review. Bug in theory, but
cannot find a locale has 'b' and 'B' in the 12 date/time style
canned pattern.

Bug: v8:8877
Change-Id: I126fdf979e8ea5b97773c8fd974359a7e52f18e9
Reviewed-on: https://chromium-review.googlesource.com/c/1480922Reviewed-by: Frank Tang <ftang@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59928}

src/objects/js-date-time-format.cc

@@ -1008,12 +1008,21 @@ icu::UnicodeString ReplaceSkeleton(const icu::UnicodeString input,
   }
   for (int32_t i = 0; i < input.length(); i++) {
     switch (input[i]) {
+      // We need to skip 'a', 'b', 'B' here due to
+      // https://unicode-org.atlassian.net/browse/ICU-20437
       case 'a':
+        V8_FALLTHROUGH;
+      case 'b':
+        V8_FALLTHROUGH;
+      case 'B':
         // ignore
         break;
       case 'h':
+        V8_FALLTHROUGH;
       case 'H':
+        V8_FALLTHROUGH;
       case 'K':
+        V8_FALLTHROUGH;
       case 'k':
         result += to;
         break;