Remove preview fallback to source view
Remove preview fallback to source view

 - Errors are handled as part of _htmlPreview.
 - image/svg is already handled as an image as part of PreviewFactory.createPreview.
 - Text types are already handled as source as part of PreviewFactory.createPreview (and with better defaults like auto-formatting).

The only branch in which case this was still hit is Other resource type with non-encoded content, but it's best not to use that as heuristic since backend might encode text content as well and vice versa, so it's better not to show any preview in that case at all. Note that user can still check raw Response tab if they wish.

Change-Id: Icb76f2d54641a73aeef62d3e1c98c731ee894ab5
Reviewed-on: https://chromium-review.googlesource.com/1216303
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Commit-Queue: Ingvar Stepanyan <604d57d65e8bfa56d804ce9639e87f7f148ac601@cloudflare.com>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#593673}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1147ea0fc184d0224e3bdf09d3be575050ea5699
ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend