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/+/2197283Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>