GitLab

Remove the ModernLinker since it becomes unused. Also remove the possibility to
set the linker differently in tests.

Public document explaining the motivation: https://goo.gl/kWnHYr

BUG=719977 (google-internal)

Change-Id: I5b9b85817093b36127fa4a607bced57b4350a5b8
Reviewed-on: https://chromium-review.googlesource.com/538582
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517838}