Traces: --offscreen syncs on N-2 frame GPU completion This avoids the issue described on the bug where an app's assumption about double buffering can create a race. This prevents the replay from getting too far ahead rendering to offscreen framebuffers. Also correct mOffscreenFramebuffers to use mTotalFrameCount as the use of mOffscreenFrameCount appears accidental. Bug: angleproject:370089935 Change-Id: I30f438eb66201fe77d61710cbe6e90d47e839dd8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5895974 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Roman Lavrov <romanl@google.com>