Remove Vulkan validation layers from .gitignore

third_party/vulkan-validation-layers/src used to be populated by a DEPS
file in ANGLE. Because of this, it was added to the .gitignore to
prevent excess code downloading.

However, ANGLE later changed to pull its DEPS into
third_party/angle/third_party. That left
third_party/vulkan-validation-layers/src inside .gitignore without
actually being pulled in by DEPS.

This patch removes that directory from the .gitignore file so we can
later update the directory. This is needed to pull in a fix for a bug.

BUG=843346

Change-Id: I0563deadeef759af11f85dbdb0781238e82b21eb
Reviewed-on: https://chromium-review.googlesource.com/1252622Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595216}

third_party/.gitignore

@@ -211,7 +211,6 @@
 /usrsctp/usrsctplib
 /v8-i18n
 /valgrind
-/vulkan-validation-layers/src
 /visualmetrics
 /wayland/src
 /wayland-protocols/src