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

BUG=skia:
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Review URL: https://codereview.chromium.org/1724493002
diff --git a/src/gpu/vk/GrVkDescriptorPool.h b/src/gpu/vk/GrVkDescriptorPool.h
index ebbf230..b20b018 100644
--- a/src/gpu/vk/GrVkDescriptorPool.h
+++ b/src/gpu/vk/GrVkDescriptorPool.h
@@ -23,7 +23,7 @@
         }
 
         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 @@
     typedef GrVkResource INHERITED;
 };
 
-
-#endif
\ No newline at end of file
+#endif