Comment out reading of platform gypi in the GN build.

This value is currently unused and causes errors in the latest version of GN.

TBR=eseidel

Review URL: https://codereview.chromium.org/294983003

git-svn-id: svn://svn.chromium.org/blink/trunk@174400 bbb929c8-8fbe-4397-9dbb-9b2b20218538