Revert of [ES6 modules] ModuleMap::GetFetchedModuleScript to return n…

…ullptr when entry not found / "fetching" (patchset #5 id:80001 of https://codereview.chromium.org/2860993002/ )

Reason for revert:
This CL probably causes test failures
 external/wpt/html/semantics/scripting-1/the-script-element/module/import-subgraph-404.html

The first failed build:
https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Trusty%20Leak/builds/4742

Original issue's description:
> [ES6 modules] ModuleMap::GetFetchedModuleScript to return nullptr when entry not found / "fetching"
>
> Before this CL, we asserted that ModuleMap::GetFetchedModuleScript always queried load completed module scripts, but it was not always the case.
> This CL changes the member function to return nullptr if the entry doesn't exist or the entry is being fetched.
>
> This CL fixes a crash that happens when we early-exit module script graph fetch by a sub-graph instantiation failed, but there are still partial subgraph in flight.
>
> BUG=718442,594639
>
> Review-Url: https://codereview.chromium.org/2860993002
> Cr-Commit-Position: refs/heads/master@{#472192}
> Committed: https://chromium.googlesource.com/chromium/src/+/1f92cfbdedaecb97e8a87773deb32f98c6997351

TBR=domenic@chromium.org,module-dev@chromium.org,neis@chromium.org,hiroshige@chromium.org,kouhei@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=718442,594639

Review-Url: https://codereview.chromium.org/2886123002
Cr-Commit-Position: refs/heads/master@{#472402}