Remove dangerous constructor from SkArenaAlloc

Make the constructors for SkArenaAlloc unambiguous.

TBR=mtklein@google.com

Change-Id: Iaa6419574e78e17bf746f5f9bcdf99c9d6c9d727
Reviewed-on: https://skia-review.googlesource.com/13540
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
diff --git a/tests/ArenaAllocTest.cpp b/tests/ArenaAllocTest.cpp
index c27c202..cb2ae0e 100644
--- a/tests/ArenaAllocTest.cpp
+++ b/tests/ArenaAllocTest.cpp
@@ -68,7 +68,7 @@
         created = 0;
         destroyed = 0;
 
-        SkArenaAlloc arena{nullptr, 0};
+        SkArenaAlloc arena{0};
         REPORTER_ASSERT(r, *arena.make<int>(3) == 3);
         Foo* foo = arena.make<Foo>(3, 4.0f);
         REPORTER_ASSERT(r, foo->x == 3);
@@ -121,7 +121,7 @@
         created = 0;
         destroyed = 0;
         std::unique_ptr<char[]> block{new char[1024]};
-        SkArenaAlloc arena{block.get(), 1024};
+        SkArenaAlloc arena{block.get(), 1024, 0};
 
         REPORTER_ASSERT(r, *arena.make<int>(3) == 3);
         Foo* foo = arena.make<Foo>(3, 4.0f);