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1) Favor --renderer-cmd-prefix more heavil when generating flags files
for ClusterFuzz.
2) Remove the --gpu-launcher option as it no longer works. Looking at
existing issues I'm not sure that this is too important for our
coverage, so it seems easier to remove it than to try to continue
maintaining it.

R=inferno@chromium.org

Bug: 904773
Change-Id: I3f0f468337540dfc5af73289ad0a46e41dcdb5ae
Reviewed-on: https://chromium-review.googlesource.com/c/1340978Reviewed-by: Abhishek Arya <inferno@chromium.org>
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609703}