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/viewer/Viewer.cpp b/tools/viewer/Viewer.cpp
index d84aa51..6c43340 100644
--- a/tools/viewer/Viewer.cpp
+++ b/tools/viewer/Viewer.cpp
@@ -19,6 +19,7 @@
 #include "src/core/SkImagePriv.h"
 #include "src/core/SkMD5.h"
 #include "src/core/SkOSFile.h"
+#include "src/core/SkReadBuffer.h"
 #include "src/core/SkScan.h"
 #include "src/core/SkTSort.h"
 #include "src/core/SkTaskGroup.h"