DevTools: Hide not-found sourcemap refs
DevTools: Hide not-found sourcemap refs

When a JS source file refers to a sourcemap, which then refers to a file
that is not found on the file system (file://), the underlying request
handler doesn't return any headers. With no headers, the DevTools UI
bindings component presumes that the response is equivalent to a 200 OK
and returns a blank file. Instead, we're going to return a 404, which
will then prevent DevTools from surfacing the not-found file altogether.

Bug: 941035
Change-Id: Icf92bdcfb415f0381217547dac9037d53915e3fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847833
Reviewed-by: Andrey Kosyakov <03b08926e49d80331ab322c2b9e0332109fbfd39@chromium.org>
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Commit-Queue: Robert Paveza <7787129de3d0cb4911c68d958db3023863584560@microsoft.com>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#704396}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9cfe83f2a243896b4db9bc463ce6c9b69e4f59e8

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