[code coverage] Add additional env vars for coverage cq trybot

This CL adds additional env vars for running webkit_layout_tests on cq
coverage trybot so that coverage metadata can be properly emitted.

Bug: 929245
Change-Id: I7d597de0a4a937504719a5383abe6f10d76391fa
Reviewed-on: https://chromium-review.googlesource.com/c/1456865
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629731}
3 files changed