Fix access of 'from-font' underline thickness

FontMetrics::UnderlineThickness() returns an Optional<float>, but this
code was dereferencing the value without checking if there was one
(using value()). This made the following check of the Optional<>
dead/redundant.

Instead receive the Optional<> without dereferencing it. Also use
value_or(...) to return the auto value or the specified value depending
on if the latter exists or not.

Bug: 785230
Change-Id: Id85ac107600ef26f2edc738048abe657b9db3bf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2238932Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#777292}

third_party/blink/renderer/core/paint/text_painter_base.cc

@@ -361,12 +361,9 @@ float ComputeDecorationThickness(const ComputedStyle* style,
 
   if (style_thickness.IsFromFont()) {
     base::Optional<float> underline_thickness_font_metric =
-        font_data->GetFontMetrics().UnderlineThickness().value();
-
-    if (!underline_thickness_font_metric)
-      return auto_underline_thickness;
-
-    return std::max(1.f, underline_thickness_font_metric.value());
+        font_data->GetFontMetrics().UnderlineThickness();
+    return std::max(1.f, underline_thickness_font_metric.value_or(
+                             auto_underline_thickness));
   }
 
   DCHECK(!style_thickness.IsFromFont());