Similarly to a prior patch this organizes the common perf test sources
into a single target. This will allow us to more easily support setting
compiler flags on particular perf test sources.

This also fixes some build errors that popped up when new warnings were
enabled.

Bug: angleproject:3630
Change-Id: I866885199f93a5055e8d160e39f72d1f8209bc75
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2103086
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>