Add explicit to ctor

Change-Id: I4232d23e0c2dfe46695e948c46f2745c8e7e555c
Reviewed-on: https://skia-review.googlesource.com/c/172945
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>

Author: herbderby

include/private/SkArenaAlloc.h

@@ -65,7 +65,7 @@ class SkArenaAlloc {
 public:
     SkArenaAlloc(char* block, size_t blockSize, size_t firstHeapAllocation);
 
-    SkArenaAlloc(size_t firstHeapAllocation)
+    explicit SkArenaAlloc(size_t firstHeapAllocation)
         : SkArenaAlloc(nullptr, 0, firstHeapAllocation)
     {}