Includes methods for creating temporary files, deleting files, and
reading files into a string. Also renames GetPathSeparator to mention
it's only used for environment variables. Includes a new virtual type
angle::Process that will be used to implement cross-platform async
Process launching for tests. Also includes a way to specify a custom
crash handler callback.

Also adds a few unit tests for the new functionality. They are disabled
on Android because the functions are not needed by the new test runner.

Bug: angleproject:3162
Change-Id: I3e2c2e9837608884c98379fa0f78c9ffbe158d73
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1821940
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>