The perf jit support assumes that if a code object has source position entries,
then it has a source file associated with them. However, the WasmToJS wrapper
stubs are exceptions to this rule which causes a crash when using `--perf-prof`
with asm.js or WASM code.

Change-Id: I047e229477844bf5357c8553ee50e22c089ab1c2
Reviewed-on: https://chromium-review.googlesource.com/897643Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#51118}