Vulkan: Add missing initialization for mTotalMemorySize

Looks like a bad merge.

Bug: b/230538246
Change-Id: I66869ebb856606dc6acd5da4e919024efe62aa79
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3635159
Commit-Queue: Amirali Abdolrashidi <abdolrashidi@google.com>
Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com>
Commit-Queue: Charlie Lao <cclao@google.com>

src/libANGLE/renderer/vulkan/vk_helpers.cpp

@@ -2749,6 +2749,7 @@ BufferPool::BufferPool()
       mHostVisible(false),
       mSize(0),
       mMemoryTypeIndex(0),
+      mTotalMemorySize(0),
       mNumberOfNewBuffersNeededSinceLastPrune(0)
 {}