Adding a sandboxed file system via `window.webkitRequestFileSystem` to
hosted mode enables using local overrides in hosted mode. Since e2e-
tests run in hosted mode, this enables adding e2e tests for local
overrides. This CL adds a first such e2e-test testing a simple response
header override.
Bug: 1313436
Change-Id: I6eb3e2ff9de336fa62648647fe48042009743d18
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3620364
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Commit-Queue: Wolfgang Beyer <wolfi@chromium.org>
