Require SK_DECLARE_STATIC_LAZY_PTR is used in global scope.

Function- or method- local scope isn't threadsafe; the pointer is generally
zero-initialized on first use in function scope (i.e. lazily... we have to go
deeper), but for globals we can be pretty sure the linker will do that for us.

BUG=skia:

No public API changes.
TBR=reed@google.com

Review URL: https://codereview.chromium.org/651723003
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index f8f44d0..96d6046 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -260,8 +260,9 @@
     return SkNEW_ARGS(DiscardableMemoryPool, (size, mutex));
 }
 
+SK_DECLARE_STATIC_LAZY_PTR(SkDiscardableMemoryPool, global, create_global_pool);
+
 SkDiscardableMemoryPool* SkGetGlobalDiscardableMemoryPool() {
-    SK_DECLARE_STATIC_LAZY_PTR(SkDiscardableMemoryPool, global, create_global_pool);
     return global.get();
 }