Adding derived sources for platform.

Pending moves from core/platform to platform have a dependency on
derived sources. Add support in this changelist.

R=abarth@chromium.org, abarth, eseidel
BUG=297477

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=159354

Review URL: https://codereview.chromium.org/26697002

git-svn-id: svn://svn.chromium.org/blink/trunk@159389 bbb929c8-8fbe-4397-9dbb-9b2b20218538