Add maxGlyphCount to SkGlyphRunList

Change-Id: I2121c4c68f4124b6a65ca9c1883f3ee0e6d89695
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244997
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>

src/core/SkGlyphRun.h

@@ -88,6 +88,13 @@ public:
         }
         return glyphCount;
     }
+    size_t maxGlyphCount() const {
+        size_t glyphCount = 0;
+        for(const auto& run : fGlyphRuns) {
+            glyphCount = SkTMax(glyphCount, run.runSize());
+        }
+        return glyphCount;
+    }
     bool allFontsFinite() const;
 
     SkPoint origin() const { return fOrigin; }