The two-pass approach first uses ANGLE's shader translator followed by
glslang. The doc explains it in more detail with links.

Bug: angleproject:3345
Change-Id: I04fd31993e3cd62ac409f2696e18f7ec39e5c6ce
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1560252Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>