Fix shading opt out in io/google-cloud-platform

[aviemzur]

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[coveralls]

Changes Unknown when pulling dd1b001 on aviemzur:fix-google-io-shading-opt-out into ** on apache:master**.

[aviemzur]

R: @amitsela
This is a fix for broken master build.

Looks like the opt-out instructions in PR #2096 (comment) were faulty and caused master to break.
I've since edited the comment with a correct version which is the one in this PR.

[amitsela]

LGTM