Assert complex precondition in SkGlyphCache::Visit.

src/core/SkGlyphCache.cpp

Index: src/core/SkGlyphCache.cpp
diff --git a/src/core/SkGlyphCache.cpp b/src/core/SkGlyphCache.cpp
index 9725c0179a73941b1ab5d2757670363e7d2bf577..072541d14681e33f8b41726c9e9b2972ec3cf8da 100644
--- a/src/core/SkGlyphCache.cpp
+++ b/src/core/SkGlyphCache.cpp
@@ -520,6 +520,16 @@ SkGlyphCache* SkGlyphCache::VisitCache(SkTypeface* typeface,
     }
     SkASSERT(desc);
 
+    // Precondition: the typeface id must be the fFontID in the descriptor
+    SkDEBUGCODE(
+        uint32_t length;
+        const SkScalerContext::Rec* rec = static_cast<const SkScalerContext::Rec*>(
+            desc->findEntry(kRec_SkDescriptorTag, &length));
+        SkASSERT(rec);
+        SkASSERT(length == sizeof(*rec));
+        SkASSERT(typeface->uniqueID() == rec->fFontID);
+    )
+
     SkGlyphCache_Globals& globals = get_globals();
     SkGlyphCache*         cache;