Fix legacy shaper to include next character after ZWJ

This patch fixes legacy word shaper to shape the next
character of ZWJ together.

Bug: 1121420
Change-Id: Ic4d3a71aa836dc8e7027e18d60772abb32964935
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388360Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803732}

third_party/blink/renderer/platform/fonts/shaping/caching_word_shape_iterator.h

@@ -125,10 +125,16 @@ class PLATFORM_EXPORT CachingWordShapeIterator final {
     bool has_any_script = !Character::IsCommonOrInheritedScript(ch);
     for (unsigned next_end = end; end < length; end = next_end) {
       ch = text_run_.CodepointAtAndNext(next_end);
-      // ZWJ and modifier check in order not to split those Emoji sequences.
+      // If ZWJ, include the next character.
+      if (ch == kZeroWidthJoinerCharacter) {
+        if (next_end < length)
+          text_run_.CodepointAtAndNext(next_end);
+        continue;
+      }
+      // Modifier check in order not to split those Emoji sequences.
       if (U_GET_GC_MASK(ch) & (U_GC_M_MASK | U_GC_LM_MASK | U_GC_SK_MASK) ||
-          ch == kZeroWidthJoinerCharacter || Character::IsModifier(ch) ||
-          Character::IsEmojiTagSequence(ch) || ch == kCancelTag)
+          Character::IsModifier(ch) || Character::IsEmojiTagSequence(ch) ||
+          ch == kCancelTag)
         continue;
       // Avoid delimiting COMMON/INHERITED alone, which makes harder to
       // identify the script.

third_party/blink/renderer/platform/fonts/shaping/caching_word_shaper_test.cc

@@ -231,6 +231,21 @@ TEST_F(CachingWordShaperTest, SegmentEmojiZWJCommon) {
   ASSERT_FALSE(iterator.Next(&word_result));
 }
 
+TEST_F(CachingWordShaperTest, SegmentEmojiZWJ) {
+  // ZWJ should include the next character in the "word", so  that they are
+  // shaped together.
+  String str(u"\U0001F3F4\u200D\u2620\uFE0F");
+  TextRun text_run(str);
+
+  scoped_refptr<const ShapeResult> word_result;
+  CachingWordShapeIterator iterator(cache.get(), text_run, &font);
+
+  ASSERT_TRUE(iterator.Next(&word_result));
+  EXPECT_EQ(str.length(), word_result->NumCharacters());
+
+  ASSERT_FALSE(iterator.Next(&word_result));
+}
+
 TEST_F(CachingWordShaperTest, SegmentEmojiPilotJudgeSequence) {
   // A family followed by a couple with heart emoji sequence,
   // the latter including a variation selector.