Cache (size / units_per_em) in HarfBuzzFontData

SkTypeface::getUnitsPerEm() is not a fast function. This
patch caches (size / units_per_em) in HarfBuzzFontData.

Improves long-line-nowrap.html from 580ms to 547ms (~6%)

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I4decd9f66e6b99b4798b1c7cc522999bba372aba
Reviewed-on: https://chromium-review.googlesource.com/1226773
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591385}

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

@@ -211,10 +211,9 @@ static hb_position_t HarfBuzzGetGlyphHorizontalKerning(
   int32_t kerning_adjustments[1] = {0};
 
   if (typeface->getKerningPairAdjustments(glyphs, 2, kerning_adjustments)) {
-    SkScalar upm = SkIntToScalar(typeface->getUnitsPerEm());
-    SkScalar size = hb_font_data->paint_.getTextSize();
     return SkiaTextMetrics::SkiaScalarToHarfBuzzPosition(
-        SkIntToScalar(kerning_adjustments[0]) * size / upm);
+        SkIntToScalar(kerning_adjustments[0]) *
+        hb_font_data->SizePerUnit(*typeface));
   }
 
   return 0;

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

@@ -83,6 +83,14 @@ struct HarfBuzzFontData {
     }
   }
 
+  float SizePerUnit(const SkTypeface& typeface) const {
+    if (size_per_unit_ != kInvalidFallbackMetricsValue)
+      return size_per_unit_;
+    int units_per_em = typeface.getUnitsPerEm();
+    size_per_unit_ = paint_.getTextSize() / units_per_em;
+    return size_per_unit_;
+  }
+
   scoped_refptr<OpenTypeVerticalData> VerticalData() {
     if (!vertical_data_) {
       DCHECK_NE(ascent_fallback_, kInvalidFallbackMetricsValue);
@@ -101,7 +109,7 @@ struct HarfBuzzFontData {
 
   // Capture these scaled fallback metrics from FontPlatformData so that a
   // OpenTypeVerticalData object can be constructed from them when needed.
-  float size_per_unit_;
+  mutable float size_per_unit_;
   float ascent_fallback_;
   float height_fallback_;