devtools: Fix distances when no user roots have been found
devtools: Fix distances when no user roots have been found

In the case that user roots are not provided, distances should start
at 0. This is the case when full heap snapshots are provided with
the gn flag
  v8_enable_full_heap_snapshots

For production builds this flag is always false.

V8 CL: https://crrev.com/c/1528994

Bug: 936797
Change-Id: Ifd247a082a4336b59d05f444ba54d281f9eae805
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529096
Reviewed-by: Alexei Filippov <52e51c43e283548b762e3c9dd9a4600768db9fb4@chromium.org>
Reviewed-by: Ulan Degenbaev <56dd103d017ceadb708086e2f118997b62b54fda@chromium.org>
Commit-Queue: Michael Lippautz <366b911b1713540eac38e45e389ba1ab59b56004@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#642413}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a48e236e02572f3841c0a43dfc6ea2eb50a47213
ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend