SkPDF: implement SkPDFArray::reserve.

SkTArray::reserve() was implemented here:
    https://skia.googlesource.com/skia/+/06ca8ec%5E%21/#F28

Change-Id: Ibe78cd686506ccb424a827b37bca9c2904565f85
Reviewed-on: https://skia-review.googlesource.com/20518
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>

src/pdf/SkPDFTypes.cpp

@@ -271,8 +271,7 @@ void SkPDFArray::drop() {
 int SkPDFArray::size() const { return fValues.count(); }
 
 void SkPDFArray::reserve(int length) {
-    // TODO(halcanary): implement SkTArray<T>::reserve() or change the
-    // contstructor of SkPDFArray to take reserve size.
+    fValues.reserve(length);
 }
 
 void SkPDFArray::emitObject(SkWStream* stream,