restricted_trace_perf: Write summary for single renderer For use when generating spreadsheets focused on Vulkan. Previously the script would exit without generating a CSV file unless there were two renderers to compare. Test: restricted_trace_perf --renderer vulkan Bug: b/317411251 Change-Id: Ic9872599edb38c738112265b1ce7f252cf3f966a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5508315 Reviewed-by: Roman Lavrov <romanl@google.com> Commit-Queue: Cody Northrop <cnorthrop@google.com>