Placeholder crates

In some circumstances tools/crates/run_gnrt.py vendor will not vendor an actual crate. When Chromium doesn't really depend on a given crate, then a placeholder crate will be generated instead. This typically happens when either:

• The crate is a conditional dependency that is:
  • Only present on certain target platforms (ones not supported by Chromium build system as determined by tools/crates/gnrt/condition.rs)
  • Only present if certain crate features are enabled (ones determined to be disabled in Chromium after crate feature resolution performed by gnrt, guppy, and cargo)
• or gnrt_config.toml asked to remove the crate (e.g. via remove_crates entry of gnrt_config.toml)

gnrt generates a placeholder crate, because otherwise cargo metadata, cargo tree, and other cargo commands wouldn't work.