If --latest-toolchain is used, SimpleChrome stage still uses an older toolchain, not the newly built one.

As a result, any SimpleChrome issues are not caught in toolchain upgrade testing. This will also catch issues related to standalone toolchain.

e.g. https://bugs.chromium.org/p/chromium/issues/detail?id=837794 and https://bugs.chromium.org/p/chromium/issues/detail?id=837889 would have been caught early.