fix unsigned/signed warning on linux for GrVkDescriptorPool::numPoolSizes()

src/gpu/vk/GrVkDescriptorPool.h

Index: src/gpu/vk/GrVkDescriptorPool.h
diff --git a/src/gpu/vk/GrVkDescriptorPool.h b/src/gpu/vk/GrVkDescriptorPool.h
index ebbf230708d55a615d2b32c515cd3300b5ee9747..b20b01827ca8aac38b553bb6f7566fad5bdc7ae2 100644
--- a/src/gpu/vk/GrVkDescriptorPool.h
+++ b/src/gpu/vk/GrVkDescriptorPool.h
@@ -23,7 +23,7 @@ public:
         }
 
         void setTypeCount(VkDescriptorType type, uint8_t count);
-        uint32_t numPoolSizes() const;
+        int numPoolSizes() const;
 
         // Determines if for each i, that.fDescriptorTypeCount[i] <= fDescriptorTypeCount[i];
         bool isSuperSet(const DescriptorTypeCounts& that) const;
@@ -52,5 +52,4 @@ private:
     typedef GrVkResource INHERITED;
 };
 
-
-#endif
+#endif