Set the "mutable format" bit on sRGB images at creation time.

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 deee409e6f6186e189127440dee24f3620c67c39..94b765f668329c6f99922bb1c7c3de1c1674562b 100644
--- a/include/gpu/vk/GrVkTypes.h
+++ b/include/gpu/vk/GrVkTypes.h
@@ -30,6 +30,10 @@
  * Vulkan textures are really const GrVkImageInfo*
  */
 struct GrVkImageInfo {
+    /**
+     * If the image's format is sRGB (GrVkFormatIsSRGB returns true), then the image must have
+     * been created with VkImageCreateFlags containing VK_IMAGE_CREATE_MUTABLE_FORMAT_BIT.
+     */
     VkImage        fImage;
     VkDeviceMemory fAlloc;    // can be VK_NULL_HANDLE iff Tex is an RT and uses borrow semantics
     VkImageTiling  fImageTiling;