Vulkan backend has a barrier tracker that tracks memory barrier needs of
all shared resources. Because the buffer/texture objects are shared
resources within a shared group, the tracker can not live in a context.
Putting it in a device/renderer requires locks. It fits perfectly in a
shareGroup object. The work is already done at API level to handle the
mutex lock for shared context access so that no extra lock needs to be
taken in the backend. This CL adds egl::ShareGroup class that represents
the object that are shared among all share context group. At the front
end this usually will include all the shared resource managers (not done
in this CL). The ShareGroup object is accessible from gl::State object.
This CL also adds ability for backend driver to allocate implementation
specific ShareGroupImpl object. Vulkan backend will then use it to keeps
the barrier tracker and other things that naturally fits the share group
concept.

Bug: angleproject:4664
Change-Id: Ifcd975cbdf5130022e21c41397894afc28f572e7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2217252Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>