Fix the ninja target -> GN label mapping for wtf_unittests, webkit_unit_tests
Fix the ninja target -> GN label mapping for wtf_unittests, webkit_unit_tests

This was causing issues when generating the build files on the bots
after flipping the blink bots to GN.

TBR=brettw@chromium.org
BUG=432959

Review URL: https://codereview.chromium.org/1508303004 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#364250}

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