gpu: Add gles2_conform_test target to GN build.

The internal stuff will come later.

Tested on Linux with the following command lines:

$ gn gen out-gn
$ ninja -C out-gn gles2_conform_test

BUG=432959
TEST=see above
R=dpranke@chromium.org,sievers@chromium.org

Review URL: https://codereview.chromium.org/1038243002

Cr-Commit-Position: refs/heads/master@{#322573}

gpu/config/BUILD.gn

@@ -52,6 +52,7 @@ source_set("config") {
 
   deps = [
     "//base",
+    "//gpu/command_buffer/service",
     "//third_party/re2",
     "//ui/gl",
   ]

gpu/gles2_conform_support/BUILD.gn

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/allocator.gni")
+import("//testing/test.gni")
 
 # GYP version: gpu/gles2_conform_support/gles2_conform_support.gyp:gles2_conform_support
 executable("gles2_conform_support") {
@@ -36,3 +37,14 @@ executable("gles2_conform_support") {
     deps += [ "//base/allocator" ]
   }
 }
+
+test("gles2_conform_test") {
+  sources = [
+    "gles2_conform_test.cc",
+  ]
+  deps = [
+    "//base",
+    "//gpu/config",
+    "//testing/gtest",
+  ]
+}