Add back clang requirement assert for use_clang_coverage flag.

R=brettw@chromium.org,mmoroz@chromium.org

Bug: 771718, 759794
Change-Id: I77d5f83587777d8ed4b23cf00d8f55ae48015ce0
Reviewed-on: https://chromium-review.googlesource.com/701619
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507165}