SkPDF: add drop() virtual to release resources early.

src/pdf/SkPDFFont.cpp

Index: src/pdf/SkPDFFont.cpp
diff --git a/src/pdf/SkPDFFont.cpp b/src/pdf/SkPDFFont.cpp
index ac9843370f6a0bd8c8dfcf0e483ad8c181a8fdd1..9e784618dab7411291e4c6a06e309f80860e6e28 100644
--- a/src/pdf/SkPDFFont.cpp
+++ b/src/pdf/SkPDFFont.cpp
@@ -1434,3 +1434,10 @@ bool SkPDFFont::CanEmbedTypeface(SkTypeface* typeface, SkPDFCanon* canon) {
     }
     return *canon->fCanEmbedTypeface.set(id, canEmbed);
 }
+
+void SkPDFFont::drop() {
+    fTypeface = nullptr;
+    fFontInfo = nullptr;
+    fDescriptor = nullptr;
+    this->SkPDFDict::drop();
+}