[tests] Ignore console line numbers without source files

Decrease scheduler flakes of layout tests which can incorrectly infer
console message line numbers depending on execution scheduling. Do this
by ignoring line numbers of console messages which do not have an
associated discovered source file.

Bug: 894792
Bug: 894795
Bug: 896194
Change-Id: Ic8565ba7ac4a2e036f9d333c319544efdc8a3601
Reviewed-on: https://chromium-review.googlesource.com/c/1283037
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600380}
82 files changed