[test] Make perf runner able to pass extra flags to d8.
[test] Make perf runner able to pass extra flags to d8.

TEST=tools/run_perf.py --arch x64 --extra-flags="--turbo --turbo-verify-allocation --turbo-greedy-regalloc" /path/to/SunSpider.json
BUG=484208
LOG=n
TBR=tandrii@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1128933007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#28384}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh