Add discard support to Vulkan

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2104353002

Review-Url: https://codereview.chromium.org/2104353002
diff --git a/src/gpu/vk/GrVkCaps.cpp b/src/gpu/vk/GrVkCaps.cpp
index 833dc24..cf9f4f0 100644
--- a/src/gpu/vk/GrVkCaps.cpp
+++ b/src/gpu/vk/GrVkCaps.cpp
@@ -25,7 +25,7 @@
     fNPOTTextureTileSupport = true;  // always available in Vulkan
     fTwoSidedStencilSupport = true;  // always available in Vulkan
     fStencilWrapOpsSupport = true; // always available in Vulkan
-    fDiscardRenderTargetSupport = false; //TODO: figure this out
+    fDiscardRenderTargetSupport = true;
     fReuseScratchTextures = true; //TODO: figure this out
     fGpuTracingSupport = false; //TODO: figure this out
     fCompressedTexSubImageSupport = false; //TODO: figure this out