Support use of non-coherent memory allocations in Vulkan.

include/gpu/vk/GrVkTypes.h

Index: include/gpu/vk/GrVkTypes.h
diff --git a/include/gpu/vk/GrVkTypes.h b/include/gpu/vk/GrVkTypes.h
index 782cb90b21ed8dffe28fc2e71cea19326d890063..aa1334adca09c1d1d313fd36857b1ae3f45a7406 100644
--- a/include/gpu/vk/GrVkTypes.h
+++ b/include/gpu/vk/GrVkTypes.h
@@ -34,6 +34,11 @@ struct GrVkAlloc {
     VkDeviceMemory fMemory;  // can be VK_NULL_HANDLE iff Tex is an RT and uses borrow semantics
     VkDeviceSize   fOffset;
     VkDeviceSize   fSize;    // this can be indeterminate iff Tex uses borrow semantics
+    uint32_t       fFlags;
+
+    enum Flag {
+        kNoncoherent_Flag = 0x1,   // memory must be flushed to device after mapping
+    };
 };
 
 struct GrVkImageInfo {