e.g. https://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device-xcode-clang/builds/72413 says that it ran off of rev 463235 - however, when it tried to generate build files, it apparently used the contents of a BUILD.gn file from one revision before with a tree at that revision (and fails)...

P-0 cuz all the trybots are red, and the tree is unhappy as well