Clean up overlay RenderPass count reporting. This fixes the trace perf test to accurately report how many RPs in each frame. Instead of counting the RPs on a flush we now count only on a swap call. This won't work for offscreen surfaces which is fine - the overlay doesn't really have the same use for offscreen rendering. Also ignores the first frame in graph data so we can ignore the first setup frame in the trace tests. Also skips the redundant extra "flush" call that would generate an empty space in the RP graph. Gives a cleaner measurement for optimizing the XFB RP count. Bug: angleproject:4622 Change-Id: I5762c500cdb216700247095984ae62b4f8741602 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2215309 Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>