The GetPositionInfo function only operates on WasmCompiledModule, so it
should be a method of that class.
This CL also splits the method in two, such that I can reuse the
GetContainingFunction method for breakpoint support.

R=titzer@chromium.org
BUG=chromium:613110

Review-Url: https://codereview.chromium.org/2521293002
Cr-Commit-Position: refs/heads/master@{#41191}