Removed Vulkan TODO for LayerTreeHostTilesTest with LowBitDepth

Bug: 987278
Change-Id: I59c71cecba1d0e14d1977230148db0bde477a1b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728431
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682786}

cc/trees/layer_tree_host_pixeltest_tiles.cc

@@ -238,7 +238,9 @@ TEST_P(LayerTreeHostTilesTestPartialInvalidationMultiThread, FullRaster) {
 using LayerTreeHostTilesTestPartialInvalidationLowBitDepth =
     LayerTreeHostTilesTestPartialInvalidation;
 
-// TODO(crbug.com/963446): Enable these tests for Vulkan.
+// This test doesn't work on Vulkan because on our hardware we can't render to
+// RGBA4444 format using either SwiftShader or native Vulkan. See
+// crbug.com/987278 for details
 INSTANTIATE_TEST_SUITE_P(
     ,
     LayerTreeHostTilesTestPartialInvalidationLowBitDepth,