Fixed an ordering issue found by UBSan_vptr.

Fixed an ordering issue found by UBSan_vptr.

We managed to access the scope_ member of CompilationInfo before its
containing object was actually constructed.

Rule of thumb: When constructing an object, never ever pass around
pointers to members which come later in the member initializer list,
you simply can't see locally if this might cause trouble or not.

Review URL: https://codereview.chromium.org/796363002

Cr-Commit-Position: refs/heads/master@{#25796}