libstdc++ does not allow incomplete type for T2 with std::pair<T1,T2>
and fails with:

.../../src/libANGLE/renderer/vulkan/vk_cache_utils.h:1570:64:
required from here
/usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/include/g++-v9/bits/stl_pair.h:215:11:
error: std::pair<_T1, _T2>::second has incomplete type
  215 |       _T2 second;     /// @c second is a copy of
                                  the second object
      |           ^~~~~~

https://chromium-review.googlesource.com/c/angle/angle/+/2580111
added class FramebufferCache with incomplete type in |mPayload| to
vk::FramebufferHelper. Changing include order is not an option.
However, FramebufferCache is only used in FramebufferVk and we
can make it local there.

Bug: chromium:957519
Change-Id: I5fbdca23adbb9f4aecc266988c02fb0d051504cb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2621473Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>