To avoid a large performance impact from adding Vulkan debug-util
markers for every GLES entrypoint, the EVENT() macro needs a run-time
check that avoids constructing a ScopedPerfEventHelper object.

Test: angle_perftests -v --local-output --gtest_filter="TracePerfTest.Run/*nba*"
Bug: b/170249632
Change-Id: I422111cdf6f6f713800e7ac587e66582bd00359f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2491009
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>