GN: Add write_runtime_deps variable Allows targets to use runtime_deps at build-time. BUG=593416 Review URL: https://codereview.chromium.org/1804303004 Cr-Original-Commit-Position: refs/heads/master@{#384411} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 0f8ad42ac1a6b159655c51bf7095ff2d21ae540f