Mac: Disable warnings about having no GLContextCGL

ImageTransportSurfaceOverlayMac is written under the assumption that it
is only ever paired with a GLContextCGL. This assumption has recently
been broken. The ERROR here was, as a result, creating an unruly amout
of console spam.

Bug: 866520
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I3f3524f3f45e9e4cdaae09aeb672c9ab3b8edafa
Reviewed-on: https://chromium-review.googlesource.com/1147100Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577211}

gpu/ipc/service/image_transport_surface_overlay_mac.mm

@@ -111,7 +111,10 @@ void ImageTransportSurfaceOverlayMac::ApplyBackpressure() {
   // https://crbug.com/863817
   CGLContextObj current_context = CGLGetCurrentContext();
   if (!current_context) {
-    LOG(ERROR) << "No context current!";
+    // TODO(ccameron): ImageTransportSurfaceOverlayMac assumes that it is only
+    // ever created with a GLContextCGL. This is no longer the case for layout
+    // tests, and possibly in other situations.
+    // https://crbug.com/866520
     return;
   }