VmaVulkanFunctions was passed to vmaCreateAllocator without initializing
all members. This leads to potential reads of uninitialized variables.

BUG=chromium:1078652

Change-Id: Id349c8fbb2dd3a5a2c64c6a017add3d6f97b78fd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2185191Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>