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/+/2215309Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>