Revert of Remove unused FontPlatformDataHarfBuzz::emSizeInFontUnits

Source/platform/fonts/harfbuzz/FontPlatformDataHarfBuzz.h

Index: Source/platform/fonts/harfbuzz/FontPlatformDataHarfBuzz.h
diff --git a/Source/platform/fonts/harfbuzz/FontPlatformDataHarfBuzz.h b/Source/platform/fonts/harfbuzz/FontPlatformDataHarfBuzz.h
index 0753a8b79f7ad4531c0f7bab076c963a63e2c99c..48f5e87fb1d30f85aedeaa288305b4c36e18f553 100644
--- a/Source/platform/fonts/harfbuzz/FontPlatformDataHarfBuzz.h
+++ b/Source/platform/fonts/harfbuzz/FontPlatformDataHarfBuzz.h
@@ -93,6 +93,7 @@
 
     unsigned hash() const;
     float size() const { return m_textSize; }
+    int emSizeInFontUnits() const;
 
     FontOrientation orientation() const { return m_orientation; }
     void setOrientation(FontOrientation orientation) { m_orientation = orientation; }
@@ -132,6 +133,7 @@
     RefPtr<SkTypeface> m_typeface;
     CString m_family;
     float m_textSize;
+    mutable int m_emSizeInFontUnits;
     bool m_syntheticBold;
     bool m_syntheticItalic;
     FontOrientation m_orientation;