Fix ideographic baseline to match spec. (#6422)

* Fix ideographic baseline to match spec.

Author: GaryQian

third_party/txt/src/txt/paragraph.cc

@@ -736,10 +736,8 @@ void Paragraph::Layout(double width, bool force) {
         max_line_spacing = line_spacing;
         if (line_number == 0) {
           alphabetic_baseline_ = line_spacing;
-          // TODO(garyq): Properly implement ideographic_baseline_ and update
-          // tests.
           ideographic_baseline_ =
-              (metrics.fUnderlinePosition - metrics.fAscent) * style.height;
+              (metrics.fDescent - metrics.fAscent) * style.height;
         }
       }
       max_line_spacing = std::max(line_spacing, max_line_spacing);

third_party/txt/tests/paragraph_unittests.cc

@@ -1876,7 +1876,7 @@ TEST_F(ParagraphTest, BaselineParagraph) {
   GetCanvas()->drawLine(0, paragraph->GetAlphabeticBaseline(),
                         paragraph->GetMaxWidth(),
                         paragraph->GetAlphabeticBaseline(), paint);
-  ASSERT_DOUBLE_EQ(paragraph->GetIdeographicBaseline(), 70.180000305175781);
+  ASSERT_DOUBLE_EQ(paragraph->GetIdeographicBaseline(), 79.035003662109375);
   ASSERT_DOUBLE_EQ(paragraph->GetAlphabeticBaseline(), 63.305000305175781);
 
   ASSERT_TRUE(Snapshot());