Revert "ServiceWorker: Implement ES modules for ServiceWorker scripts behind"

This reverts commit 260d4608eb584984dd073c8a2a7d4dac2a0058f6.

Reason for revert: Suspect that it is causing MSan failures, see https://crbug.com/883302.

Original change's description:
> ServiceWorker: Implement ES modules for ServiceWorker scripts behind
> the test-only flag
> 
> This CL allows to import and evaluate as a module script at
> ServiceWorker. Users can use it by adding type 'module' when
> register a service worker script.
> 
> As the first step, this CL implements ES Modules for new ServiceWorker
> scripts. ES Modules for installed ServiceWorker scripts will be
> implemented in separate CLs. Also, this doesn't implement dynamic
> import yet. This feature is enabled by the test-only flag in this CL.
> 
> i.e.
> // index.html
> const registration = await navigator.serviceWorker.register(
>                          "service_worker.js",
>                          { type: "module" });
> 
> Spec: https://w3c.github.io/ServiceWorker/#serviceworkercontainer-interface
> 
> Bug: 824647
> Change-Id: I023c7ca881a90327730011699a8db879653cafdf
> Reviewed-on: https://chromium-review.googlesource.com/1194809
> Commit-Queue: Asami Doi <asamidoi@google.com>
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
> Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#590628}

TBR=falken@chromium.org,kinuko@chromium.org,kouhei@chromium.org,nhiroki@chromium.org,asamidoi@google.com

Change-Id: I33d0220d687716e7a63c84170a3a6bafa97cc988
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 824647, 883302
Reviewed-on: https://chromium-review.googlesource.com/1221704
Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590719}