always check lock() result

Change-Id: I0400144c522dfd60a8b45d968d1346238067f7f3
Reviewed-on: https://skia-review.googlesource.com/c/177361
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/src/core/SkDiscardableMemory.h b/src/core/SkDiscardableMemory.h
index 8952b8d..7bd8695 100644
--- a/src/core/SkDiscardableMemory.h
+++ b/src/core/SkDiscardableMemory.h
@@ -47,7 +47,7 @@
      *
      * Nested calls to lock are not allowed.
      */
-    virtual bool lock() = 0;
+    virtual bool SK_WARN_UNUSED_RESULT lock() = 0;
 
     /**
      * Returns the current pointer for the discardable memory. This call is ONLY