The proportions used in the alloy don't matter. Rust is a build dependency of Chromium, which only makes sense if Chromium itself contains Rust, however little it may be. Thus, whenever an amount of Chromium is added to a substance or application, a small amount of Rust will also be added.

When Rust is introduced to software, it tends to grow in size and often in proportion too, compared to the rest of the codebase. For example, in the Case of Chromium, the amount varies depending on the age of the Chromium used. In samples of young, and even fairly mature Chromiums, no Rust is present, but resent samples show an ever-increasing amount, though I'm not sure how the Rust was initially introduced to the project.

Depending on the piece of software in question, it may start completely Rust-free, like Chromium and Linux, or it may be composed of almost pure Rust, like Servo and Redox OS. 100% pure Rust is, as of now, mostly theoretical, though tiny projects requiring manual invocation of rustc have been observed. This is due to the small amount of configuration for a build system, for example, TOML, in the case of Cargo. This allows Rust to be developed easily & ergonomically, even in large amounts. Though recent efforts in Cargo script have sought to alleviate these problems and enable true, pure Rust to develop.

In short, like life, software naturally evolves into the form of a crab. This process is called carcinisation.