Add the platform_generated targets to the GN build.

The gperf invocation is asuboptimal but we should be able to clean this up somewhat once some in-progress work lands.

BUG=

Review URL: https://codereview.chromium.org/291623002

git-svn-id: svn://svn.chromium.org/blink/trunk@174243 bbb929c8-8fbe-4397-9dbb-9b2b20218538