Revert "Flatten ModuleTreeLinker"

This reverts commit 5a168df.

Reason for revert: Seems to be breaking ModuleTreeLinkerTest.FetchTreeWith3Deps1Fail on:
https://uberchromegw.corp.google.com/i/chromium.memory/builders/Linux%20TSan%20Tests/builds/11696

Original change's description:
> Flatten ModuleTreeLinker
> 
> 1. This CL applies the spec change by
> whatwg/html#2971.
> Since [1], the code has already been behaving like the spec PR 2971, and this CL
> makes the code align with PR 2971 more completely, by removing AncestorList
> and adding spec comments.
> 
> 2. This CL also flattens the code structure of ModuleTreeLinker, which is
> enabled by the spec PR 2971, to make the code simpler to make further
> optimizations easy.
> That is, instead of creating a ModuleTreeLinker for each module script
> (i.e. for each "fetch the descendants" call) in a module graph, this CL
> creates a single ModuleTreeLinker that corresponds to a top-level module graph
> script.
> 
> Most of fetching-related classes/methods, including
> - ModuleTreeLinker::DependencyModuleClient
> - ModuleTreeLinker::FetchDescendants()
> - ModuleTreeLinker::NotifyOneDescendantFinished()
> are merged into the single method NotifyModuleLoadFinished() that implements
> the main body of "fetch the descendants" and
> "internal module script graph fetching procedure".
> 
> This also removes ModuleTreeReachedUrlSet and instead uses HashSet<KURL>
> directly, as we no longer have to share it across multiple ModuleTreeLinkers.
> Modulator::FetchTreeInternal() is removed as we no longer create
> ModuleTreeLinkers for subtree fetching via Modulator.
> 
> 3. This CL applies a part of
> whatwg/html#2991, particularly introduces
> FindFirstParseError() that corresponds to "find the first parse error" and
> use it to find the error to be reported, instead of propagating errors
> based on Step 6.1 and 6.2 of "fetch the descendants".
> 
> (*) In some subtle cases, this might cause behavior changes in error reporting,
> but these changes shouldn't be significant, because anyway the spec before
> PR 2991 (and thus the previous implementation) behaves nondeterministically
> in some similarly subtle cases.
> 
> This CL is an intermediate step to apply spec PRs 2971 and 2991.
> This CL refactors largely ModuleTreeLinker with keeping the existing behavior
> mostly (except for (*)), and subsequent CLs will apply the behavior changes
> and remaining code structure changes introduced by PR 2991.
> 
> Bug: 763597
> Change-Id: I0ef38c5ebf462fa7f02093f1725ea0014b80585d
> Reviewed-on: https://chromium-review.googlesource.com/583552
> Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
> Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#503034}

TBR=hiroshige@chromium.org,ksakamoto@chromium.org,kouhei@chromium.org,nhiroki@chromium.org

Change-Id: I84c8f8f443bdbb1d5fbe8cb52d2cb513c7890d2c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 763597
Reviewed-on: https://chromium-review.googlesource.com/674510
Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503063}