This reflects both the contract in blink, as well as what we
plan to do in streamed compilation, where we'll want to lay out
bytes received such that each section and each function body is
contiguous, but they may all be separate - which entails a copy.

BUG=chromium:697028

Review-Url: https://codereview.chromium.org/2797653002
Cr-Commit-Position: refs/heads/master@{#44387}