Add testonly to blink_devtools_frontend_resources_files

This target depends on the DevTools `all_devtools_files` target. We are
in the progress of marking that target as testonly.

`blink_devtools_frontend_resources_files` can be testonly itself, as
`blink_web_tests` (which is the only target that depends on it) is
marked as testonly itself.

Bug: 1011811
Change-Id: Ic128457f5dba06992401e6c6d43550df1fa8c12a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095063
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748720}

third_party/blink/public/BUILD.gn

@@ -456,6 +456,7 @@ group("blink_devtools_frontend_resources") {
 }
 
 group("blink_devtools_frontend_resources_files") {
+  testonly = true
   data_deps = [ "$devtools_frontend_path:devtools_all_files" ]
 }