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"Contributing" doc has moved from dev.chromium.org to Markdown file,
update the link.

Bug: None
Change-Id: I8574dec9fb980b347a024f067c0aa59637e6eaca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678286Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
Commit-Queue: Xiaochu Liu <xiaochu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672658}