Reduce indentation in a section of build/common.gypi a bit.
Reduce indentation in a section of build/common.gypi a bit.

Use the common formatting for conditionals instead of something custom in a
block of code I'm about to touch for something else.  No behavior change.

BUG=none

Review URL: https://codereview.chromium.org/1842863002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#383793}

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