Fixes the collection of V8.WasmFuncctionSizeBytes.wasm for UMA stats.

Bug: v8:6852
Change-Id: Ib25d249dd2856ffb8a3205e54ba052c1bc9a09cf
Reviewed-on: https://chromium-review.googlesource.com/678448Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48125}