The call to isolate_->AdjustAmountOfExternalAllocatedMemory in
WasmCodeManager::FreeNativeModuleMemories can cause a GC, which can
indirectly call WasmCodeManager::FreeNativeModuleMemories again. It
seems that this recursive call can cause memory to be deallocated
twice. With this CL we clear the list of owned_memory after all entries
were deallocated so that we cannot deallocate them again.

I think this CL fixes a crash we saw on ChromeCrash. I don't know how
to reproduce the issue though, or how to write a test for it.

R=mstarzinger@chromium.org

Bug: chromium:812532
Change-Id: I3b66274f9b72919952a4211e984192c0867a6c22
Reviewed-on: https://chromium-review.googlesource.com/921226Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51312}