To support future work where RendererVk functionality is moved to
ContextVk.  Given multiple contexts, EGLSync can no longer rely on a
single serial as it can be used in multiple contexts.  Instead, the
fence corresponding to the submission in which the EGLSync object
signals is kept so it can be waited on.

Introduces a `vk::Shared` class that includes a ref-counted reference to
a Vulkan object (vk::Fence in this case).  This is specially made to
`destroy()` object when reference count reaches zero.

Bug: angleproject:2464
Change-Id: I68c8229eea8df77974e28fcc2a9563dae5d204f9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1493131
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill (use @chromium please) <jmadill@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>