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

src/pdf/SkPDFFont.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <ctype.h>
 
 #include "SkData.h"
@@ skipping 1416 matching lines @@
     sk_sp<const SkAdvancedTypefaceMetrics> fontMetrics(
             face->getAdvancedTypefaceMetrics(
                     SkTypeface::kNo_PerGlyphInfo, nullptr, 0));
     if (fontMetrics) {
         canEmbed = !SkToBool(
                 fontMetrics->fFlags &
                 SkAdvancedTypefaceMetrics::kNotEmbeddable_FontFlag);
     }
     return *canon->fCanEmbedTypeface.set(id, canEmbed);
 }
+
+void SkPDFFont::drop() {
+    fTypeface = nullptr;
+    fFontInfo = nullptr;
+    fDescriptor = nullptr;
+    this->SkPDFDict::drop();
+}