This prepares switching the gcov coverage bot to GN.

We skip instrumenting test executables explicitly in gn configs.
In gyp, we did the same through an extra compiler wrapper script.

NOTRY=true

Bug: chromium:645890
Change-Id: I663fb479347063ae9228598d356bb654ca2a496c
Reviewed-on: https://chromium-review.googlesource.com/548275
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46217}