The driver uniform code is used by various tree_ops functions. But all
driver uniform code are defined and implemented in the
TranslatorVulkan.cpp file. There is dependency rule that tree_ops code
can not call into vulkan specific function. Right now we are working
around this problem by always having TranslatorVulkan creates uniform
and pass it down to tree_ops. This creates inefficiency for cases that
dFdy/dFdx where we don't know if we will need the driver uniform or not,
until we walk the tree and see the dFdx node. This CL refactors driver
uniform code into its own file and class under tree_utils so that
everybody can use. Mean time we can also make it much easier for metal
to expand it to have its own uniform structure.

Bug: b/173047182
Change-Id: I06bd9a005ccd6dc0a21321a3010dda1eab9d6fdb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2533443
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>