Perf: Detect context used by trace Capturing supports multiple contexts, which are hard coded into the function and file names. On desktop, they've typically been "1", but on Android we've seen context "2" or "3". This CL adds the ability to detect the context number used by the trace, and programmatically adds it to generated files. Test: angle_perftests --gtest_filter="*Trace*" Bug: b/152512564 Bug: angleproject:4036 Change-Id: I64616b93a704446b08cb614b2a74ab1932ef1f40 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2197283 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Commit-Queue: Cody Northrop <cnorthrop@google.com>