[mathml] Properly initialize StretchyOperatorShaper::Metrics

Properly initialize StretchyOperatorShaper::Metrics in the struct,
instead of having to initialize the struct at point of use. This comes
out of a discussion of this CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2270448

Bug: 6606
Change-Id: I1f00753ea25ce8de18e853729cb90e72a7e04e61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275954Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#785306}

third_party/blink/renderer/core/layout/ng/mathml/ng_math_radical_layout_algorithm.cc

@@ -120,7 +120,7 @@ scoped_refptr<const NGLayoutResult> NGMathRadicalLayoutAlgorithm::Layout() {
         std::max(horizontal.kern_after_degree, -index_inline_size);
   }
 
-  StretchyOperatorShaper::Metrics surd_metrics = {};
+  StretchyOperatorShaper::Metrics surd_metrics;
   if (HasBaseGlyphForRadical(Style())) {
     // Stretch the radical operator to cover the base height.
     StretchyOperatorShaper shaper(kSquareRootCharacter,

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

@@ -31,10 +31,10 @@ class PLATFORM_EXPORT StretchyOperatorShaper final {
       : stretchy_character_(stretchy_character), stretch_axis_(stretch_axis) {}
 
   struct Metrics {
-    float advance;
-    float ascent;
-    float descent;
-    float italic_correction;
+    float advance { 0.0f };
+    float ascent { 0.0f };
+    float descent { 0.0f };
+    float italic_correction { 0.0f };
   };
   // Shape the stretched operator. The coordinates of the glyph(s) use the same
   // origin as the rectangle assigned to the optional OUT Metrics parameter.