Migrate PersistentCacheUtils logic into a cpp file.

This code is not performance-sensitive, and there doesn't seem to be any
benefit to making it header-only.

Change-Id: Ic83be5cbb96f08c18a144954be7453aaabaa5a72
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/394900
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/tools/gpu/MemoryCache.cpp b/tools/gpu/MemoryCache.cpp
index 242ea4a..70fedea 100644
--- a/tools/gpu/MemoryCache.cpp
+++ b/tools/gpu/MemoryCache.cpp
@@ -7,6 +7,7 @@
 
 #include "include/utils/SkBase64.h"
 #include "src/core/SkMD5.h"
+#include "src/core/SkReadBuffer.h"
 #include "src/gpu/GrPersistentCacheUtils.h"
 #include "tools/gpu/MemoryCache.h"