This isn't strictly necessary, as one can always build with the earlier
cdproto version specified in go.mod. However, many people still install
chromedp in GOPATH via 'go get -u', so this workaround makes life easier
for a lot of developers.
Fixes#285.
tip is rather unstable, so we shouldn't block PRs if it happens to break
our build or tests.
While at it, run 'go mod tidy' with the latest tip version.
