BUILD: add jpeg_encode_accelerator_unittests on CrOs CQ bots

This CL adds jpeg_encode_accelerator_unittests to the chromiumos_preflight
target, so it gets compiled as part of the CQ (chromeos-amd64-generic-rel
bot).

Bug: 732531
Change-Id: I49e2db7d8259ee7746100cd858d1d19638384e87
Reviewed-on: https://chromium-review.googlesource.com/c/1297249Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602193}

BUILD.gn

@@ -862,6 +862,9 @@ if (is_chromeos) {
         "//media/gpu:video_encode_accelerator_unittest",
       ]
     }
+    if (use_vaapi) {
+      deps += [ "//media/gpu:jpeg_encode_accelerator_unittest" ]
+    }
   }
 }