Right now the dynamic buffer for default uniform is per program. Most of
time the buffer is unused. That is a huge waste of memory (and these
memory are wired memory). This CL moves the mDefaultUniformStorage from
per ProgramVk to ContextVk so that we all share with each other.

Bug: b/161391337
Change-Id: I1fe8523b2b2dbc39bec3509a3432e38e34bd5713
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2274870Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Charlie Lao <cclao@google.com>