Vulkan: Refactor DynamicBuffer::allocate. Reduces duplicate code by re-using allocateFromCurrentBuffer. Bug: angleproject:6980 Change-Id: I3f1126a53136be8f68a3a79008dd9adae45c86b8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3449279 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>